Learn Croatian: Ova mjesta su lijepa. - These places are beautiful.

You are right that feminine singular words usually end in -a. However, this sentence is teaching the plural of neuter nouns! The singular word for place is mjesto (neuter). In the plural, regular neuter nouns change their ending to -a. Any adjectives or pronouns describing them also take the -a ending.

Since ova mjesta looks exactly like a feminine singular phrase, how do I know it means plural places?

The biggest clue in the sentence itself is the verb su (are). If this were a singular feminine subject, the verb would be je (is). You also need to learn the base noun: knowing that place is mjesto tells you it is a neuter noun, so mjesta must be its plural form.

Is ova the plural form of ovo?

Exactly! Ovo means this for a singular neuter noun, like ovo mjesto (this place). Because we are talking about multiple places, we use the neuter plural form ova to mean these.

What is the base form of the adjective lijepa here?