An ELT Glossary : The Infinitive

The infinitive is a non-finite verb  (ie not marked for person, tense or number) which may be used :

a) alone - the "bare" infinitive : I might go.

b) preceded by the infinitive particle toI want to go now.

As these examples show, the form of the infinitive is the base form (ie the uninflected form) of the verb. It can be negated by being preceded by the negative particle not :  I'd rather not see him: I told you not to do that!

The infinitive, with or without the infinitive particle, is used :

a) after modal auxiliaries and the primary auxiliary Do  : We can go now; He does complain a lot! 

b) after some other catenative verbsI want to go now; We helped (them) (to) clear up; I heard him come in; They made her do it

c) to express purpose, where it can be seen as an ellipted version of in order to + infinitive: I'm going there to see David; I'm phoning to ask about Saturday; To get to Greenwich, you can take a train or go by boat; We stopped to have lunch.

d) in expressions where, again, it can be seen as an ellipted version of a full expression - this time be going toThe Queen is to visit America = The Queen is going to visit America;

d) as a non-finite clause acting as subject or object of another verb : To err is human ; The best thing would be to ask about it;

e) as a sentence adverbial (disjunct) : To be honest, I've known about it for a while;  or connective expression (conjunct) : To conclude, I'd like to say that....

f) as a non-finite clause post-modifying a noun phrase : The best time to arrive would be about 11; John is the person to ask about that; 

g) After be or have to express obligation or necessity : You are to be back here by 8pm; We have to leave tomorrow. 

h) After various adjectives following some copulative verbs : He was about to leave; He seemed certain to pass; It's great to be here; He's bound to win; it feels good to be home; The authorities appeared powerless to intervene; He looks likely to run in the election; He's apt to exaggerate; The theory proved difficult to verify; It sounded sure to fail.

i) As a non-finite clause following too or enoughDon't be too quick to believe what he says; I've got too much work to do; He was speaking too quietly to be heard; I don't feel strong enough to go out; He didn't express himself clearly enough to convince me; I don't have enough money to go; I don't know enough to decide.

For some great teaching ideas...

Gerngross, Puchta and Thornbury, Teaching Grammar Creatively, CUP

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