Learn Dutch: Ze zijn thuis. - They are at home.

The verb tells us! zijn is the plural form of the verb 'to be' (meaning 'are'). If the sentence was about 'she', it would use the singular verb is: Ze is thuis (She is at home).

Why use ze instead of zij?

ze is the unstressed version of zij ('they'). Dutch speakers use short, unstressed forms like ze in normal conversation unless they want to add emphasis. If you wanted to emphasize that they are at home (and not someone else), you would use zij.

Wait, I thought zijn meant 'his'?

It can mean both! zijn is the plural verb for 'are', but it is also the word for 'his'. You can tell them apart by the context. Here, it acts as the main verb right after the subject ze.

Why is it just thuis? Do we need a word for 'at'?

You do not! The single Dutch word thuis already means the entire phrase 'at home'. It comes from an old combination of words that meant 'at house', which over time merged into the single word thuis.