Learn Croatian: Pisma su stara. - The letters are old.

That is the tricky part about neuter nouns! In the singular, the word for a letter is pismo, which is neuter. When regular neuter nouns become plural, their ending changes to -a. So even though pisma looks like a feminine singular word, it is actually a neuter plural word.

Why is the adjective stara used here instead of stari?

Adjectives must always match the gender and number of the noun they describe. Because pisma is a neuter plural noun, the adjective also takes the neuter plural ending, which is also -a. This makes it stara.

Since pisma and stara both end in -a, how can I tell if a sentence is feminine singular or neuter plural?

The verb is your best clue! Here, we have the plural verb su (are), which tells you we are talking about more than one thing. If this were a feminine singular subject, you would see the singular verb je (is) instead.

Does pisma mean letters or the letters?

Croatian does not have words for a or the. Pisma can mean either letters or the letters depending on the context. Here, it makes the most sense in English to translate it as the letters because we are talking about specific letters being old.