Learn English – What does “deepest summer” mean

Conventionally because is considered a conjunction, because it links (or conjoins) two clauses. The argument against its classification as a conjunction is that because operate like other subordinating or coordinating conjunctions but does operate like prepositions.

This post on the Language Log demonstrates that because is not a conjunction by comparing it to that (subordinating conjunction) and and (coordinating conjunction). I summarize the argument below.

Typical subordinating conjunction: that

• That can be omitted from a sentence and the sentence's meaning generally remains the same.
• The clauses cannot be rearranged and maintain meaning: "[clause 1] that [clause 2]" != "that [clause 2], [clause 1]".
• Complements follow that.
• If because is omitted from a sentence the meaning does not remain the same.
• With because the clauses can be rearranged.
• Adjuncts generally follow because.

Typical coordinating conjunction: and

• The two clauses joined by and can be rearranged and the meaning stays the same.
• And cannot start a sentence.
• Rearranging the clauses changes the meaning with because.
• Because can start a sentence.

Because does not act like conjunctions, and thus is not a conjunction. It does, however, act like prepositions.

Prepositions can introduce noun-phrases, clauses, preposition phrases, and nothing, depending on the preposition. Because has long introduced clauses and preposition phrases (with of) and more recently also introduces noun-phrases.