The infinitive form of a verb is usually preceded by "to" (e.g., to run, to dance, to think). The word "to" is not a preposition in this case; it is often called the "sign of the infinitive". An infinitive can be used as a noun, an adjective or an adverb.
To dance was his ambition. (as a noun)
His ambition was to dance. (as a noun)
They need water to drink. (as an adjective)
The officer returned to help. (as an adverb)
After certain verbs the "to" is dropped.
I helped her tidy the balcony. ("to" dropped from "to tidy")
They made him wait for a day. ("to" dropped from "to wait")
Glossary of grammatical terms