Learn Russian: Второй поезд задержали из-за аварии. - The second train was delayed because of the accident.

Потому что translates to "because" and connects two full clauses with a subject and a verb. Из-за means "because of" or "due to" and is a preposition followed directly by a noun. It is often used for negative or external causes, like an accident.

Why is the word for accident spelled аварии?

The preposition из-за requires the Genitive case. The dictionary form is авария, which is a feminine noun ending in -ия. In the Genitive case, feminine nouns ending in -ия change their ending to -ии.

The translation says "was delayed", but isn't задержали the plural "they delayed"?

Yes! Russian frequently uses the third-person plural past tense without a pronoun to express a passive idea. Literally, this sentence says "They delayed the second train..." which is the most natural way to express "The second train was delayed" when the specific person who did the delaying isn't important.

What case is Второй поезд in, and why is it at the beginning of the sentence if it is not the subject?

It is in the Accusative case because it is the direct object (the thing being delayed). Since поезд is an inanimate masculine noun, its Accusative form looks exactly like its Nominative form. It sits at the beginning because Russian word order usually puts known information (the second train) first, and the new action or reason at the end.