Learn Romanian: Păsările sunt în copac. - The birds are in the tree.

In Romanian, the word for 'the' attaches to the end of the noun. The singular word for bird is pasăre, its plural is păsări, and to say 'the birds' (feminine plural), you add -le to the end, making păsările.

Doesn't sunt mean 'I am'?

It means both 'I am' and 'they are'! The verb a fi (to be) shares the exact same form for the first-person singular and the third-person plural. Because the subject here is Păsările (the birds), we know it translates to 'are'.

Why does it say în copac instead of în copacul for 'in the tree'?

In Romanian, after most simple prepositions like în (in), pe (on), or la (at/to), you usually drop the attached 'the' from the end of the noun. Even though we translate it as 'in the tree', in Romanian you just use the base word and say în copac.