Core Patterns

Modal Auxiliaries

can, could, may, might, must, shall, should, will, would: nine helping verbs called modal auxiliaries, or modals. A modal changes the kind of claim a sentence makes: an obligation or a possibility instead of a fact. Two of them, will and shall, also make the future tense.

What you’ll learn

Name the nine modal auxiliaries and the meanings each one adds
Put a base form after a modal, and avoid the common mistakes with “to”, “-s” and “do”
Keep a modal, its helpers and the word “not” together in the verb slot
Find the subject and the verb phrase of a question that starts with a modal
Explain why the dependency tree splits a verb phrase that the diagram keeps whole

What a modal adds

A modal does not describe an action — it comments on it. She leaves states a fact; She must leave states an obligation; She might leave states a possibility. The action is identical in all three.

Each modal adds its own meaning, and most of them have more than one:

can: ability or permission. Omar can read music.
could: past ability, a polite request or a possibility. Could you speak more slowly?
may: permission or a possibility. You may start the test now.
might: a possibility, often less sure than with may. Nina might come later.
must: an obligation, or a strong belief that something is true. Passengers must wear a seat belt. The baby must be hungry.
shall: an offer or a suggestion in a question, or the future in formal English. Shall I carry your bag?
should: advice, or what is expected. You should drink more water.
will: the future, a prediction or a promise. I will send you the photos tonight.
would: a polite request or offer, an imagined situation, or the future seen from the past. Would you like some tea?

Spanish often uses poder where English chooses among can, could, may and might, and deber where English chooses between must and should.

Only will and shall can give a verb phrase a tense: the future. With any other modal, the verb phrase has no tense in this app. An offer or a suggestion such as Shall I carry your bag? has no tense either.

Because the modal is part of the verb, it goes in the verb slot. It is not a modifier, so it never hangs below the base line on a slant. The deletion test shows the difference: you can delete an adjunct, and the sentence still stands. Delete must from She must leave and you get She leave, which is not correct English. The verb has to change to leaves, and the obligation is gone.

“She must leave.”

Reed-Kellogg — the modal is part of the verb

nsubj Nominal subject
aux Auxiliary
ROOT Root of the sentence

Dependency — the modal depends on the main verb

Comparing the two views

Reed-Kellogg writes must leave as one text in the verb slot, because the modal is part of the verb. The dependency tree splits the two words, but it too treats must as a helper, not a modifier: leave is the root, and must hangs from it as aux. Notice the direction. The modal depends on the main verb, even though it comes first.

Have to and ought to

have to means much the same as must, and ought to as should. Three things are different:

The verb phrase is only the first word: has in Diego has to take the bus. The to + verb after it is a complementary infinitive phrase. In the Roles exercise and the diagram, ought works the same way, and it is tagged MD, like the nine modals.
The Tense and Aspect filters treat them differently: has in has to take is present, like any verb phrase that starts with a present form. But ought to take is one modal verb phrase with no tense, like should take, and the helpers after to give its aspect: ought to have called is perfect.
In the negative, must and have to differ. You mustn’t park here means that parking is not allowed, but You don’t have to pay means that paying is not necessary.

Modals stack with other auxiliaries

A modal always comes first in its verb phrase, and the next verb is a base form: must leave, should wait. Only one modal fits in a verb phrase: must can go is not English. After the modal, have and be can add the perfect and the progressive: should have waited, might have been sleeping. However long the chain gets, it stays one slot.

The word not belongs in the slot too, right after the modal: should not have waited is one verb phrase, and the Roles exercise boxes it as one. In the tree, not hangs from the main verb as neg. A contraction splits into two tokens: can’t is ca + n’t, and won’t is wo + n’t. In the Tags exercise, ca and wo are MD.

Find the main verb
It is the last verb in the chain. In Nobody should have waited, it is waited.
Collect the helpers in front of it
The modal, any have or be, and not if there is one. Here: should and have.
Write the whole chain in the verb slot
Keep the sentence’s order: should have waited. The Roles exercise boxes the same three words as the Verb phrase.

“Nobody should have waited.”

Reed-Kellogg — modal plus perfect, still one slot

nsubj Nominal subject
aux Auxiliary
ROOT Root of the sentence

Dependency — should and have both hang from waited

Comparing the two views

Three words in one Reed-Kellogg slot become three nodes in the tree. waited is the root, and both should and have hang from it as aux. Each view shows something the other hides: the tree shows how each helper links to the main verb, and the diagram shows that the three words work as one unit.

Quick check

Which words in You may borrow my laptop, but you must return it tomorrow are modal auxiliaries, and what does each one add?
Answer
may and must, both tagged MD. may gives permission, and must adds an obligation. Each one starts its own verb phrase, may borrow and must return, and the verb after it is a base form.
Correct these two sentences: Do you can carry this box? and Camila must to finish her essay.
Answer
Can you carry this box? A modal makes its own question: it moves in front of the subject, so there is no do. Camila must finish her essay. After a modal, the next verb is a base form, with no to.
What goes in the verb slot of the Reed-Kellogg diagram for The guests might have taken a taxi? Which word is the root of the dependency tree?
Answer
might have taken: the modal, the helper have and the main verb share one slot, because together they do one job. The tree splits them: taken is the root, and might and have both hang from it as aux.
Turn Should we invite Elena? into a statement. What are its subject and its verb phrase? How many boxes does the Roles exercise want for the verb phrase of the question?
Answer
We should invite Elena. The subject is we, and the verb phrase is should invite, written in one slot in the diagram. In the question, we stands between Should and invite, so the Roles exercise wants two boxes: Should and invite.

Key terms

Modal auxiliary: One of nine helping verbs (can, could, may, might, must, shall, should, will, would) that add ability, permission, possibility, obligation, advice or prediction. It comes first in its verb phrase, and the next verb is a base form. Tagged MD; in the tree it hangs from the main verb as aux. You must leave now. It might rain later.
See: Tense · Aspect · Mood
Auxiliary: Any helping verb: the modals, plus be, have and do when they support a main verb rather than being one. She has finished. — has helps finished. She has a car. — here has is the main verb, not an auxiliary. One exception in the exercises: the POS exercise wants AUX for a linking be even when it is the only verb, because the parser labels it that way.
See: Subjects & Verbs · Verbs & Auxiliaries
Verb phrase: The main verb with its helping verbs, plus not or never when they go with it; its first verb is finite. A clause has one verb phrase, or one per verb when the predicate is compound. However many words it has, it fills one slot on the base line. A verbal is not a verb phrase. Some grammars use the name for the whole predicate. She had been waiting. — three words, one slot. They will have finished by noon.
See: Subjects & Verbs · Grammar Basics
Inversion: Moving an auxiliary in front of the subject to form a question. Undone in the Reed-Kellogg diagram, preserved in the dependency tree. She can swim. → Can she swim? They have left. → Have they left?
See: Sentence Types & Voice · Subjects & Verbs

Practice on a real sentence

This sentence has no modal and no auxiliary: has is the main verb, as in She has a car in the Auxiliary entry above. So in the POS exercise has is VERB, not AUX, and in Tags it is VBZ, not MD. In Dependencies it is the root, with no aux below it.

Loading the practice sentence…