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Core Patterns

Modal Auxiliaries

can, could, may, might, must, shall, should, will, would — nine small words that change what a sentence is claiming without changing what it is about. They sit in the verb slot with everything else.

What you’ll learn

  • Recognise the nine modal auxiliaries and what each adds
  • Keep a modal in the verb phrase rather than treating it as a modifier
  • Handle a modal inside a question
  • See why the dependency parse splits a verb phrase 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.

Because the modal is part of the verb, it goes in the verb slot. It is not a modifier and never hangs on a slant, even though it feels like it is adding detail. The test from Grammar Basics settles it: delete a modal and the sentence changes meaning rather than merely losing detail.

“She must leave.”

Shemust leave
Reed-Kellogg — the modal is part of the verb
nsubjauxROOTShemustleave
  • nsubjNominal subject
  • auxAuxiliary
  • ROOTRoot of the sentence
Dependency — the modal depends on the main verb

Comparing the two views

Reed-Kellogg writes must leave as one piece of text — the modal is grammatically part of the verb, so it goes where the verb goes. The dependency parse disagrees about the packaging but not the analysis: it makes leave the root and attaches must as aux. Notice the direction — the modal depends on the main verb, not the other way round, even though it comes first.

Modals stack with other auxiliaries

English allows only one modal per verb phrase — must can go is not English — but a modal happily combines with the perfect and the progressive: should have waited, might have been sleeping. However long the chain gets, it stays one slot.

A negative belongs there too. should not have waited is one verb phrase; the not is not a modifier hanging below it.

“Nobody should have waited.”

Nobodyshould have waited
Reed-Kellogg — modal plus perfect, still one slot
nsubjauxauxROOTNobodyshouldhavewaited
  • nsubjNominal subject
  • auxAuxiliary
  • ROOTRoot of the sentence
Dependency — two aux arcs onto one root

Comparing the two views

Three words in one Reed-Kellogg slot become three separate nodes in the tree — waited as root with aux arcs from both should and have. Neither view is more detailed than the other here; they are detailed about different things. The tree tells you the internal make-up of the verb phrase; the diagram tells you the verb phrase is a unit.

Modals in questions

English forms a question by moving the first auxiliary in front of the subject. When there is a modal, that is the word that moves — which leaves the subject sitting in the middle of its own verb phrase.

  1. Undo the inversion

    Could the parcel have arrived? → The parcel could have arrived.

  2. Read off the parts

    Subject parcel; verb phrase could have arrived.

  3. Diagram those parts in the usual places

    The diagram does not preserve the question’s word order — it shows the grammar, and the grammar is the same as the statement’s.

“Could the parcel have arrived?”

parcelCould have arrivedthe
Reed-Kellogg — the inversion is undone
auxdetnsubjauxROOTCouldtheparcelhavearrived
  • auxAuxiliary
  • detDeterminer
  • nsubjNominal subject
  • ROOTRoot of the sentence
Dependency — word order kept, arcs cross instead

Comparing the two views

Here the two views part company completely on word order. Reed-Kellogg un-inverts the question, putting the subject back on the left, so the diagram looks like the statement it corresponds to. The dependency tree keeps the words exactly where they appear and lets the arcs cross instead — which is why Could sits at the far left while depending on arrived at the far right.

Key terms

Modal auxiliary
One of nine verbs — can, could, may, might, must, shall, should, will, would — that add possibility, obligation or prediction. Tagged MD, and always aux in the tree. You must leave now. It might rain later.
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.

See: Subjects & Verbs

Verb phrase
The main verb plus its auxiliaries, occupying one slot on the base line however many words it contains. She had been waiting. — three words, one slot. They will have finished by noon.

See: 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: Imperatives & Conjunctions, Sentence Types & Voice

Practice on a real sentence

Look for the auxiliary. In the tag exercise it will want VBZ rather than MD — this sentence has a plain verb, not a modal — and in the dependency exercise notice which word ends up as the root.

“Santiago has a nice watch”

Parts of speech

Give every word its coarse Universal Dependencies category.

Words 0/5Attempts 0Correct 0Time 0:00

Sentence — select a part of speech below, then click the word(s) it applies to

Santiago has a nice watch