Learn Polish: Ten mężczyzna czyta.

In Polish, most nouns ending in '-a' are feminine and would normally use the feminine pronoun ta (this). However, mężczyzna (man) is a very important exception. Because it refers to a male person, it is grammatically masculine and must be paired with the masculine pronoun ten.

Does czyta mean 'reads' or 'is reading'?

It means both! Polish does not have a separate continuous tense like English does. Depending on the situation, czyta translates to either 'reads' as a general habit or 'is reading' right at this moment.

Could I just say Mężczyzna czyta without using Ten?

Yes, you could. Mężczyzna czyta simply means 'The man is reading' or 'A man is reading' because Polish does not use little words for 'a' or 'the'. Adding Ten specifically points him out, changing the meaning to 'This man'.

Why isn't there a word for 'is' before the verb?

In English, we use 'is' plus an '-ing' verb to show an action happening right now (is reading). In Polish, the verb czyta all by itself contains the entire meaning of the action. You never use the Polish word for 'is' (jest) next to an action verb to say someone is doing something.