Definition of an infinitive clause

An infinitive clause or phrase is one which contains an infinitive as the main verb.

There are two types of infinitive clause:

1. The to-infinitive clause. The verb follows 'to'. 

I need to go shopping this afternoon.

My boss allows me to leave early on Fridays.

2. A bare infinitive phrase. The verb does not follow 'to'. 

I must go shopping this afternoon.

My boss lets me leave early on Fridays.

An infinitive clause can act as a noun, an adjective or an adverb.

1.     A noun clause - the subject or object of a sentence answering the question 'who?' or 'what?'

She helped to write the report.

Her job as a coach is to teach language skills.

2.     An adjective clause modifies the noun, giving more information about it. These clauses give an opinion or a purpose for the noun.

I don't have time to go to the meeting. [modifies 'time']

John wants a book to read. [modifies 'book']

3.     An adverbial clause adds information to the main clause, answering questions such as 'when?', 'where?', 'why?', 'with what goal?' and 'under what conditions?'

John was invited to speak at the conference. [tells us why he was invited]

Jeremy is keen to improve his grammar.