Non-Finites (Infinitive, Gerund, Participle)

A non-finite verb carries no tense of its own; the word before it decides its shape.

1. Gerund (V-ing) after a preposition. Always: "good at singing", "afraid of failing", "instead of waiting". Also after these verbs: enjoy, avoid, mind, finish, suggest, consider, deny, admit, risk, practise, postpone.
2. Infinitive (to + V1) after these verbs: want, hope, decide, agree, promise, refuse, manage, expect, offer, pretend; and after an adjective: "happy to help", "too tired to walk".
3. Bare infinitive (no "to") after a modal, and after make, let, see, hear, watch + object. "He made me wait", "I saw her leave" — but the passive restores the "to": "I was made to wait."
4. Participle phrases must describe the subject. "Walking down the road, I saw an accident" is right; "Walking down the road, the accident was seen" leaves the accident doing the walking — a dangling participle.

The single most common mistake is with expressions where "to" is a preposition, not part of the infinitive: look forward to, be used to, be accustomed to, object to, with a view to, in addition to. All of them take -ing: "I look forward to meeting you", not "to meet you".

How to spot it: try putting a plain noun after the "to". If "I look forward to your letter" makes sense, then "to" is a preposition, and a preposition takes the -ing form.

preposition + V-ing · enjoy/avoid/mind/finish/suggest + V-ing · want/hope/decide/agree + to + V1 · make, let, see, hear + object + bare V1 · look forward to / used to / object to + V-ing (noun test) · participle must describe the subject

Fill in the blank: "We look forward to ____ from you at the earliest." Options: hear / hearing / heard / be heard. → hearing. In "look forward to", the "to" is a preposition, and a preposition is followed by the -ing form. Sanity check: replace the blank with a noun — "We look forward to your reply" reads perfectly, which proves "to" is a preposition here and not the start of an infinitive.