Is "Because it was raining heavily" correct?

Why

Tanya's group of words does look like a sentence — it starts with a capital letter, ends with a full stop, and has a subject ("it") and a verb ("was raining"). But "because" is a word that specifically sets up an unfinished thought — it promises a reason is coming, which means it needs to be attached to a MAIN clause that tells you what that reason explains. "Because it was raining heavily" leaves a reader hanging, silently asking "...so what happened?" — the sentence structure itself signals that something is still missing. A clause like this, which cannot stand alone because a word like "because" makes it dependent on another clause, is called a subordinate (or dependent) clause, and it always needs a main clause attached to complete the thought: "Because it was raining heavily, we stayed indoors." Test: read "Because it was raining heavily" out loud to someone and watch them wait for you to continue — that expectant pause is the fragment being felt directly. Her brother is right: a clause starting with "because" cannot stand alone as a complete sentence — it needs a main clause attached to finish the thought.