For eller fordi?

"Because" is a subjunctive and therefore begins a clause. When such a clause comes last in the sentence, we can often replace "because" with the conjunction "for" without changing the meaning. Example:

1. He stayed home today because he was not fit.
2. He stayed at home today because he was not fit.

Sentence 2 consists of two full sentences linked together by the conjunction "for". Therefore, the wording is different from the wording in the clause in sentence 1:

In a complete sentence, a phrasal adverb ("not") comes after the verb.
In a clause, a phrasal adverb ("not") comes before the verb.

In sentences like No. 1 and 2 above, "for" and "because" mean the same thing.

Sometimes we can just use FOR or just BECAUSE. When we use BECAUSE, the conjunction expresses the reason for what the parenthesis expresses:

He is crying because he has beaten himself.
Reason: He has beaten himself
Consequence: He cries

If we are not talking about cause, we must use FOR:

He must have overslept, because he didn't make it to the first class.

Here we cannot follow the pattern with "because" above:

WRONG: He must have overslept because he didn't come to the first class.
Reason: He did not come to the first class
Consequence: He must have overslept

We understand that "he didn't come to the first class" cannot be the reason why he overslept.