Is "to do something" a preposition phrase?

Question

I just learned about phrase structure, but I can’t figure out how to analyse an infinitive sentence.
For example, in the sentence "Mary tries to do her homework", I guess "try to do her homework" is a verb phrase, and "try" is the verb, but what about "to do her homework"? Considering that "to" belongs to preposition, is this a preposition phrase? However, I guess "to" in infinitive is not a preposition, so I think it should not be a preposition phrase. Therefore, I was thinking maybe "try to do" is the verb, and "her homework" is the noun phrase, but I’m really not sure. Could anyone help me with this question? Thank you!

BillJ · Accepted Answer

Mary tries [to do her homework].

"Tries to do her homework" is a verb phrase functioning as the predicate of the sentence.
"Tries" is a catenative verb and the infinitival clause "to do her homework" is its catenative complement.
As Pax comments in their answer, "to" is not a preposition here but a subordinator, a special marker for VPs of infinitival clauses.
The term 'catenative' is derived from the Latin word for "chain", for the construction is repeatable in a way that forms a chain of verbs in which all except the last has a non-finite complement. In your example the chain is a short one consisting of just the two verbs "try" and "do".

Pax · Answer

"to" is merely a marker of the infinitival clause "(to) do her homework", functioning here as complement of "tries".  The to-infinitival clause is a clausal category, not a lexical category.
In the distant past the infinitive was inflected for nominative/accusative or dative, but with the decay of inflectional endings the preposition "to" was commandeered to mark the majority of infinitives, whether they were originally dative or not.  It can therefore no longer be trusted to convey any kind of prepositional meaning, and may safely be analysed as a mere marker.

Is "to do something" a preposition phrase?

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