Learn Polish: Kiedy mam czas, czytam. - When I have time, I read.

In Polish, the verb endings already tell you who is doing the action. Both mam (I have) and czytam (I read) end in -m, which is a common ending for 'I' in the present tense. You do not need to add the pronoun ja (I) unless you want to emphasize it.

Why didn't the word czas change its ending? Isn't it the direct object of the verb?

You are right that it is the direct object, so it takes the accusative case. However, czas is a masculine inanimate noun (a thing, not a person or animal). For these types of nouns, the accusative form is identical to the basic dictionary form, so it stays exactly the same.

Could czytam also mean 'I am reading'?

Yes! Polish does not have a separate continuous tense like English does. The single word czytam can mean 'I read' as a general habit, or 'I am reading' right at this moment. You rely on the context to know which one is meant.

Is the comma strictly necessary here?

Yes. In Polish punctuation, you almost always put a comma between two clauses (parts of a sentence that each have their own verb). Because we have mam in the first part and czytam in the second, they must be separated by a comma.