What is the infinitive phrase in the sentence: "Cecil wants to lose weight."?
"to lose weight"
An infinitive is the most basic form of a verb. It takes the form "to + verb," as in "to walk," "to be," "to jump," or "to lose."
Frequently infinitives can function as nouns.
Cecil wants to lose weight.
Here, the object of the verb "wants" is the phrase "to lose weight." What does Cecil want? To lose weight.
"Lose" is a verb, but when we say "to lose weight" we are treating this phrase like a noun.
Thus this is the infinitive phrase in the sentence.