Is for an adverb?

No, for is not an adverb. The word “for” typically functions as a preposition (e.g., “She’s going on vacation for two weeks”), but it can also function as a coordinating conjunction meaning “because” (e.g., “I will go now, for it is late”).

Phrases that begin with “for” often function adverbially (e.g., “… for two weeks”). However, “for” is a preposition in these phrases.

If you use “for” as a conjunction in modern English it can sound very formal or old-fashioned. You can use QuillBot’s free Paraphraser to rephrase sentences that contain “for” as a conjunction.