Learn German: Ich besuche ihn, obwohl er krank ist. - I am visiting him although he is sick.

The word obwohl (although) is a subordinating conjunction. In German, these types of connecting words always push the conjugated verb to the very end of their clause. Here, it pushes ist to the end.

Do I always need a comma before obwohl?

Yes! In German, you must always put a comma between a main clause and a dependent clause. Whenever you use a subordinating conjunction like obwohl, weil, or dass, a comma is required to separate the two parts of the sentence.

Why is it ihn instead of er in the first part of the sentence?

The verb besuchen (to visit) needs a direct object, which must be in the accusative case. ihn is the accusative form of er (he), exactly like how we say him instead of he in English.

The translation says I am visiting, but there is no word for am in the German sentence. Why is that?

German does not have a separate continuous tense like English does. The simple present tense Ich besuche covers both I visit and I am visiting. You just rely on the context to know which one makes sense.