Learn Arabic (Modern Standard): لم يتقدم الأسطول، إذ كان البحر عاصفا. - The fleet did not advance, seeing that the sea was stormy.

While لأنّ translates directly to "because" and emphasizes a direct cause, إذ is a slightly more formal or literary conjunction meaning "since" or "seeing that." It is used to present a situational context or a known background fact that explains the main action.

Could we have used إذ إنّ instead of just إذ in this sentence?

Not in this exact structure. The particle إنّ must be followed immediately by a noun or an attached pronoun suffix. Because the next word here is the verb كان (was), we can only use إذ. To use إذ إنّ, we would have to drop the verb and say إذ إنّ البحرَ عاصفٌ (seeing that the sea is stormy), but that would change the tense of the clause.

Why is the word for stormy written as عاصفا rather than just عاصف?

It is the predicate (the descriptive part) of the verb كان (was). Verbs in the كان family require their subject to be in the nominative case (like البحر) but force their predicate into the accusative case. For an indefinite masculine word like this, the accusative is marked by adding an alif and a double fatha at the end.

Why does the verb يتقدم take the jussive mood here?

The negative particle لم (did not) always forces the following present-tense verb into the jussive mood. For a regular verb like يتقدم, this means dropping the final short vowel and replacing it with a sukoon (a silent marker), meaning it is pronounced "yataqaddam" instead of "yataqaddamu".