Learn Polish: Ponieważ była awaria, pociąg był opóźniony. - Since there was a breakdown, the train was delayed.

It is highly recommended to use ponieważ here. While bo (because) is very common in everyday speech, it sounds awkward and is generally considered incorrect at the very beginning of a sentence. When you want to start a sentence with 'because' or 'since', always choose ponieważ.

Why do we say była awaria (there was a breakdown) instead of saying the train broke down?

In Polish, it is very natural to use the noun awaria (a breakdown, failure, or malfunction) with the verb być (to be) to describe these situations impersonally. You could also say pociąg się zepsuł (the train broke down), but stating that there was a breakdown is exactly how a station announcement or official update would phrase it.

Is opóźniony an adjective or a verb in the past tense?

It is a passive participle, which functions just like an adjective here. It comes from the verb opóźnić (to delay). Because it describes the masculine noun pociąg, it takes the masculine ending -y. You will often see participles like this used with the verb być (to be) to describe a state, just like in English.

Do I always need a comma after the ponieważ clause?