Learn Italian: Si svegliano sempre presto la mattina. - They always wake up early in the morning.

The Italian verb for "to wake up" is svegliarsi, which is a reflexive verb. This means the action reflects back on the subject (literally, "to wake oneself"). Si is the reflexive pronoun for "themselves".

Since si can also mean "himself" or "herself", how do we know this means "they"?

You can tell by the verb ending! Svegliano ends in -ano, which is the "they" form for an -are verb. If it were "he wakes up" or "she wakes up", the verb would be si sveglia.

Why is sempre placed after the verb instead of before it?

In English we say "always wake up", but in Italian, adverbs of frequency like sempre (always) usually go immediately after the verb. So it translates structurally as "they wake themselves always".

Why does the sentence use la mattina instead of a word for "in" the morning?

To express "in the morning" or "in the evening" for a recurring, habitual action, Italian simply uses the definite article (la or il) plus the time of day. You don't need to add a preposition for "in".