Infinitive markers in English grammar are function words that distinguish the base forms from the infinitive forms of English verbs. Function words perform definite grammatical functions but lack definite lexical meaning.
Infinitive marker is a grammatical function.
The grammatical form that can function as the infinitive marker in English grammar is the p-word, specifically the p-word to.
Infinitive markers are constituents of the verb phrase.
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Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.