When to use the infinitive The infinitive form is used after certain verbs: - forget, help, learn, teach, train - choose, expect, hope, need, offer, want, would like - agree, encourage, pretend, promise - allow, can/can't afford, decide, manage, mean, refuse * I forgot to close the window. * Mary needs to leave early. * Why are they encouraged to learn English? * We can't afford to take a long holiday.

As Jura said the infinitive is used after certain verbs .Some other verbs not mentioned by Jura are hesitate, begin, deserve, wish, start, continue, consent, prepare, fail, ( and there are more). The infinitive is also used to express the reason for doing something. I went to the supermarket 'to buy' some milk. In this sense it is often used to answer the question 'why?'. (Why did you go to the supermarket? 'To buy some milk') The infinitive can also be used as the subject of a sentence. 'To fix a car requires a lot more than a screwdriver. 'To write in English is difficult. (we also use the verb -ing form in this way)