Learn Polish: Ponieważ jego nieobecność była długa, musiał szukać nowej pracy. - Since his absence was long, he had to look for a new job.

No, you shouldn't. While both words mean "because", bo is used colloquially and almost always comes in the middle of a sentence to connect two clauses. When you want to put the reason first, starting the sentence with "Since..." or "Because...", you must use ponieważ.

Why is długa in the nominative case instead of the instrumental? Doesn't the verb "to be" take the instrumental?

The verb być (to be) takes the instrumental case when you connect the subject to a noun (like "he is a doctor"). However, when you use a standalone adjective to describe the subject, like długa (long) describing nieobecność (absence), the adjective stays in the nominative case.

Why is nowej pracy in the genitive case?

The verb szukać (to look for) is one of the specific Polish verbs that always require their object to be in the genitive case, rather than the typical accusative case. Therefore, "to look for a new job" becomes szukać nowej pracy.

How do we know "he" had to look for a job if the pronoun on isn't in the second part of the sentence?

The past tense verb musiał (had to) has a masculine ending, which automatically tells us the dropped subject is "he". If a woman had to look for a job, the sentence would use the feminine form musiała.