Hello, Meriem—When "to" is followed by the base form a verb, it introduces an infinitival clause. Infinitival clauses function in various ways. Sometimes they complement verbs; sometimes they complement nouns; sometimes they are the subject of a sentence; and sometimes they they are purpose clauses.
The "to" that introduces an infinitival clause is NOT the preposition "to." You can call it infinitival "to"; that is how it is often spoken of in the literature. It doesn't matter whether the infinitival clause is a complement clause or a purpose clause; the grammatical nature of infinitival "to" is the same in both cases.
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