A verb can wear three different disguises — gerund, participle, infinitive — and still not be the "real" verb of the sentence. Modals like must, should and need carry precise shades of meaning that Olympiad papers test closely.
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A finite verb changes with tense and subject. A non-finite verb never does — it works as a noun, adjective or adverb instead.
📌 The three non-finite forms
Gerund (verb + -ing as a noun): "Swimming is good exercise." It can be the subject or object of a sentence.
Participle (verb form as an adjective): "The barking dog woke me." / "The broken vase lay on the floor."
Infinitive (to + verb): "She wants to travel." It names a purpose or a whole action.
✨ Same word, different job
Gerund — a noun
names the action itself
Reading is my favourite hobby.
Participle — an adjective
describes a noun
I picked up the fallen leaves.
Modals never change form and never take -s, -ed or -ing. Each one carries its own precise degree of certainty, obligation or ability.
📊 What each modal really means
| Modal | Shade of meaning | Example |
|---|---|---|
| must | strong obligation / near-certainty | You must submit the form today. |
| should | advice, not a rule | You should revise before the exam. |
| need not | no obligation (it's optional) | You need not attend; it's optional. |
| may | permission or possibility | May I leave early today? |
| might | weaker possibility than may | It might rain this evening. |
Common trap: "must not" forbids an action, but "need not" simply removes an obligation. "You must not park here" (forbidden) is very different from "You need not park here" (optional, but allowed).
Achievers questions often ask you to spot a non-finite verb hiding inside a long sentence, or to judge the strength of a modal in context.
🏆 Spotting the trap
Find the real (finite) verb first — the one that changes with tense and the subject.
Every other -ing or -ed word is doing a noun's or adjective's job, not the main verb's.
For modals, ask "how sure or how obliged is the speaker?" — that decides between must/should/may/might.
Pro tip: "Used to" (past habit) is often confused with a modal — it is not one, and it never changes: "She used to sing," not "She uses to sing."
12 IEO-pattern questions on non-finite verbs and modal auxiliaries, including two Achievers-level questions.
