Here's why:
* Conjunctions connect words, phrases, or clauses. They show relationships between the connected elements. Common conjunctions include "and," "but," "or," "because," "so," etc.
* "For" is usually used as a preposition. Prepositions show the relationship between a noun or pronoun and another word in the sentence. For example: "I went *for* a walk."
However, there are some instances where "for" can act as a conjunction:
* In formal writing, "for" can introduce a reason or explanation. This is similar to using "because." For example: "He was late, *for* he had missed the bus."
* In older English usage, "for" could act as a coordinating conjunction, meaning it joined elements of equal grammatical rank. This usage is rare today.
It's important to distinguish between the prepositional and conjunctional uses of "for." If you're unsure whether "for" is functioning as a conjunction, it's best to check its grammatical function in the sentence.