Learn Italian: Maria è assente perché si è ammalata ieri. - Maria is absent because she got sick yesterday.

We use perché when the reason comes in the middle of the sentence, linking the result (Maria is absent) to its cause (she got sick). If you want to state the cause first at the beginning of the sentence, you would use siccome: Siccome si è ammalata ieri, Maria è assente.

Can I start this sentence with perché to mean 'Because she got sick...'?

In Italian, it sounds unnatural to start a statement with perché when you mean 'because'. If you want to start the sentence with the reason, you should use siccome (Since / Because). You generally only start a sentence with perché if you are asking a 'Why?' question.

Why is it si è ammalata instead of just è ammalata?

The verb for 'to get sick' is ammalarsi, which is a reflexive verb. The si is the reflexive pronoun. If you just say è ammalata, you are using an adjective to say 'she is sick' right now. By using the reflexive pronoun and the helper verb essere, si è ammalata forms the past tense (passato prossimo) to mean 'she got sick' yesterday.

Why does ammalata end in -a?