Welcome to the forum, sarah.
Try here: The Infinitive Phrase .if you have any questions, do come back to this thread.
i need help learning what a infinitive phrase is.
I think it should be pointed out for the sake of accuracy that these notes - as practical as they are - are based on a popular definition of 'infinitive' as a two-word phrase (to + V).
Strictly speaking, however, an infinitive is a one-word form, such as 'be' in
He must be here.
, identical to the so-called 'base-form' of a verb, making such collocations as 'to be', 'to go' etc. themselves infinitive phrases (irrespective of the addition of other elements).