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Affix Hopping

Too Much in Love – Examples in Context

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The Puzzle: Verbs Gone Wild

Auxiliary verbs are those little ‘pre-verbs’ that go before main verbs. English has a number of these, including:

  • (i) be            (e.g. John is eating.)
  • (ii) have       (e.g. John has eaten.)
  • (iii) must      (e.g. John must eat.)

Instead of ‘must’, in (iii), you can also substitute any one of a group of similar words—what linguists call ‘modal’ auxiliaries—including ‘may’, ‘can’, ‘might’, ‘should’, and ‘will’. So, altogether, that’s quite a few auxiliaries.

But wait, there’s more. The three kinds of auxiliaries also combine, in certain orders, into strings of multiple auxiliaries—and that leads to a crazy number of verb forms. For example, the verb ‘eat‘ can appear in all of these ways (from 0 auxiliaries, up to 3):

  • John eats.
  • John is eating.
  • John has eaten.
  • John has been eating.
  • John must eat.
  • John must be eating.
  • John must have eaten.
  • John must have been eating.

And actually, there’s more than that too: if we include past tense forms (e.g. ‘John was eating‘), and passive forms (e.g. ‘John was being eaten‘), the whole paradigm adds up to a whopping 22 forms for each English verb! It’s verbs gone wild!

The Solution: Affix Hopping

Fortunately, the auxiliary system in English is actually a lot simpler—and a lot more interesting—than it looks on the surface. In the rest of this post, we’ll show you how it all ‘really’ works—at least according to the ideas of modern linguistics.


The key to understanding auxiliaries is to realize that they’re more than ‘stand alone’ words: auxiliaries can also influence, or make requirements of, the words that follow them. Specifically, for the auxiliaries ‘be‘ and ‘have‘:

  • Auxiliary be requires that the next word end in –ing (e.g. …be eating)
  • Auxiliary have requires that the next word end in –en or –ed (e.g. …have eaten)

To explain this, many linguists argue that we should picture these auxiliaries not as simple words, but as complex, dynamic, entities: they are words that also shape the form of the next word, when you combine them.  Under this conception:

  • Auxiliary be is really a complex word with two parts: BE -ING.  The –ing ending must ‘hop’ over onto the next word, like this:

    BE -ING (progressive) pattern illustration

    BE -ING (progressive) pattern

  • Auxiliary have is really a complex word, like this: HAVE -EN. The –en ending (which sometimes shows up as -ed, with certain verbs) must also ‘hop’ over onto the next word, like this:

    HAVE -EN (perfect) pattern illustration

    HAVE -EN (perfect) pattern

In terms of word order, neither HAVE -EN nor BE -ING is required in a sentence;  but if you have both, then HAVE -EN comes first. Linguists commonly summarize those word-order facts with a template like this:


Just by itself, that template would give the wrong order, of course—we don’t say ‘*I haveen being eat’! However, because we have specified that -EN and -ING both ‘hop’, the model actually gives just the right word-orders and endings for those parts of the pattern.  For example, observe how the two parts in this model work separately with eat, and then combine to make have been eating:

BE -ING (progressive) illustration

BE -ING (progressive) with ‘eat’

HAVE -EN (perfect) with verb 'eat'

HAVE -EN (perfect) with verb ‘eat’

HAVE -EN BE -ING (perfect + progressive) with verb 'eat' illustration

HAVE -EN BE -ING (perfect + progressive) with verb ‘eat’

Adding Tense (past vs. present)

The ‘First Come First Served’ Law of Tense

Now, verbs, as you know, normally appear with marking for either ‘present tense’ (e.g. ‘John eats‘) or ‘past tense’ (e.g. ‘John ate‘).

However, a funny thing happens when you add an auxiliary: now the verb itself will no longer display present or past marking; instead, the auxiliary will be the one that shows present vs. past marking. For example, tense is fine on the auxiliaries here:

  • John is eating. (PRES marking on be=is)
  • John was eating. (PAST marking on be=was)
  • John has eaten. (PRES marking on have=has)
  • John had eaten. (PAST marking on have=had )

But if, after an auxiliary, it was the verb that still showed its usual present/past tense marking, we would get crazy forms like this: ‘*John be eats‘, ‘*John is eatings’, or ‘*John have ate’!

Notice, further, what happens with tense if you use two auxiliaries: now only the first auxiliary, not the second, will show marking for present/past. For example, tense is fine on the first auxiliary here:

  • John has been eating. (PRES marking on have = has )
  • John had been eating. (PST marking on have = had )

But if we tried to use the present or past tense forms on the second auxiliary (or the verb again) we would get impossible forms like this: ‘*John had was eating, or ‘*John has wasen ate‘!

Descriptively, it all adds up to what Dr. Dexterous likes to call the ‘first come first served’ law for tense:

Tense marking (special form for past vs. present) shows up always (and only) on the FIRST element in a string of auxiliaries + verb.

Tense as Hopping Too

To account for this pattern, most linguists today think that English has a special ‘tense assigner’, which they sometimes label TNS.

TNS fits into the auxiliary series, and is another one of those entities, like HAVE -EN or BE -ING, that influences a following word—in this case, requiring the following word to take a tense ending, or (for irregular verbs) a special idiosyncratic tense form. We can visualize it like this (for the present tense version of TNS):

present tense pattern illustration

present tense pattern

Though this ‘tense assigning’ element is in some ways similar to HAVE -EN or BE -ING, TNS is also, as you can see, in one respect different: TNS itself is ‘invisible’. That is, though we see its effects on other words, we never see TNS as a separate word (there is no overt parallel to the ‘have‘, or ‘be‘). Apart from what it requires of the following word (‘show past marking!’ or ‘show present marking!’), the putative TNS marker itself is silent:  it has influence, but no form.

A ‘silent tense assigner’ might sound a little crazy at first, but it actually turns out to nicely predict the placement of tense marking throughout the English auxiliary system—so much so, that it’s become quite a standard assumption in linguistics. We just have to add TNS to our template, like this:


And from this, we get a linguistic model that will give us just the right word-order, tense forms, and verb endings for almost all of the auxiliary combinations. Here’s some examples showing how:

Present tense on 'eat' (simple present) illustration

Present tense on ‘eat’ (simple present)

BE -ING with verb 'eat' (progressive) illustration

BE -ING with verb ‘eat’ (progressive)

TNS HAVE -EN with verb 'eat' (perfect) illustration

TNS HAVE -EN with verb ‘eat’ (perfect)

TNS HAVE -EN BE -ING with verb 'eat' (perfect plus progressive) illustration

TNS HAVE -EN BE -ING with verb ‘eat’ (perfect plus progressive)

Adding Modals

We should also, of course, fit modals (e.g. ‘must‘) into our system—but this is now quite straightforward. Modals fit in like this:


And that’s all we need to say! Because modals are the one kind of auxiliary that doesn’t ‘do’ anything to the following word: modals just sit there, inert, in the auxiliary series. And that, along with the existing account of TNS and the other auxiliaries, captures exactly how sentences with modals are ordered:

modal with verb 'eat'

modal with verb ‘eat’

Modal BE -ING with verb 'eat' (modal plus progressive) illustration

Modal BE -ING with verb ‘eat’ (modal plus progressive)

modal HAVE -EN on verb 'eat' (modal plus perfect) illustration

modal HAVE -EN on verb ‘eat’ (modal plus perfect)

modal - HAVE -EN BE -ING (modal plus perfect plus progressive) on verb 'eat' illustration

modal – HAVE -EN BE -ING (modal plus perfect plus progressive) on verb ‘eat’

As you may have noticed, modals don’t ever change form when you assign them tense, at least in the terms of the model as we have developed it here.  This is true for both present and past. For example we don’t say *He musts…” for present tense, or ‘*He musted…” for past.

This is not true in all languages, but it appears to be general property of English modals: they are the same in past, present, and unmarked forms. Go figure.


A full account of the English auxiliary system would have to include one more element: a
‘passive-be auxiliary’. This auxiliary is different from BE -ING, and in fact can co-occur with it, as in this example (the passive ‘be‘ comes last, and is highlighted here):

John must have been being eaten.

Passive ‘be‘ actually fits elegantly into the system shown here —but Dr. Dexterous is now exhausted with auxiliaries, and will save detailed discussion for a future post. (Okay, spoiler: passive ‘be‘ takes the form BE -EN and the -EN hops.)

Linguistic History and Developments

The affix hopping model was first developed by the linguist Noam Chomsky in the late 1950’s. It was one of the first uses in linguistics of ‘movement’ within a model—an approach which has become extremely influential, and a key part of modern ‘generative’ grammar.

Linguists still use the basics of the affix hopping model today. Nowadays, linguists don’t always take the idea of the ‘hop’, in this particular case, too literally, though: most see it instead as a more abstract relation, where the verb starts out with its endings already on, and the auxiliary just ‘checks’ that the following word is in the right form. However, the original insight—that auxiliaries and tense require a certain form on the following word—remains the same.

Notes on Traditional Terms

Once we have the insights of the affix hopping model in hand, many traditional grammatical terms become easier to understand.  So in case you want to learn some grammatical lingo:

  • Progressive: is the BE VERB-ING form (for example ‘He is eating‘ is present tense progressive)
  • Perfect: is the HAVE VERB-EN form (for example ‘He has eaten‘ is present tense perfect)
  • Present Participle: is the the form the verb takes after BE -ING (for example, in be eating, eating is in the present participle form)
  • Past Participle: is the form the verb takes after HAVE -EN (for example, in have eaten, eaten is in the past participle form)

The last two terms are standard in traditional grammar, but quite misleading. Just as a ‘french fry’ has nothing do with France, a ‘present participle’ actually has nothing to do with the present tense: the ‘present’ participle does not contain the present tense, does not mean the same as ‘present’, and does not even have to be in a present tense sentence at all.  Similarly a ‘past participle’ bears no connection to past tense.  What were those grammar teachers smoking?


Chomsky, Noam. Syntactic structures. ‘s-Gravenhage: Mouton, 1957.

Syntactic structures revisited: contemporary lectures on classic transformational theory
Howard Lasnik – Marcela A.Depiante – Arthur Stepanov – MIT Press – 2000

“A Comprehensive Grammar of the English Language (General Grammar) [Hardcover].” A Comprehensive Grammar of the English Language (General Grammar): Randolph Quirk, Sidney Greenbaum, Geoffrey Leech, Jan Svartvik: 9780582517349: Books. N.p., n.d. Web. 18 Dec. 2013.


2 Responses to Affix Hopping

  1. Tereza January 31, 2015 at 7:58 am #

    Interesting post, thank you!


  2. Nancy November 6, 2015 at 8:09 pm #

    Very beneficial and interesting. And the illustrations are AMAZING!

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