Learn Turkish: Ders çalışmak zor değil.

The verb çalışmak usually means "to work" (like at a job). To specifically mean "to study" in an academic sense, Turkish uses the phrase ders çalışmak, which literally translates to "to work a lesson".

Why does çalışmak keep its -mak ending here instead of being conjugated?

The -mak or -mek ending makes the verb an infinitive. It turns the action into a noun, similar to saying "studying" or "to study" in English. Here, the action itself is the subject of the sentence.

Does ders need a suffix here, like the accusative case?

No, because you are talking about studying in general, not a specific lesson. When part of a general activity phrase like ders çalışmak, the noun stays in its bare dictionary form.

Where is the word for "is" in this sentence?

In Turkish, the present tense "is" is often implied in sentences with adjectives or nouns. The word değil negates the adjective zor, so zor değil means "not hard", with the "is" naturally understood.