Question

Locate the complete infinitive phrase and identify its use. The best topic to write about is one you know well.

Answer 1

An infinitive phrase is a group of words introduced by "an infinitive", This group of words may function as a noun, adjective or adverb. In the following example the infinitive phrase is working as an adjective, since it modifies the noun "topic".

The best topic   to write about   is one you know well.

              noun   infinitive phrase

Answer 2

"to write about" I believe is the infinitive phrase and I think its use may be the use of a noun but I am not sure