What part of speech is "for" in "I have been studying for two hours"?

In the sentence "I have been studying for two hours," "for" functions as a preposition. Prepositions are words that typically indicate relationships in terms of time, place, direction, and other connections. In this case, "for" introduces a prepositional phrase that specifies the duration of the studying—indicating that the action has been occurring over a span of two hours.

This usage is quite common in English, as prepositions help to provide more detail about the actions or states described by the verbs. The prepositional phrase "for two hours" clarifies how long the studying has lasted, thus enhancing the meaning of the sentence.