Learn Hardcore German: Der Nachbar spricht gern am Nachmittag, wenn er Zeit hat. - The neighbor likes to talk in the afternoon when he has time.

Because Der Nachbar is the subject of the sentence—the one who is doing the speaking. In German, the subject always appears in the nominative case.

Why is the verb spricht placed second in the main clause?

German follows the “verb‐second” (V2) rule for main clauses: the conjugated verb must be the second idea or element in the sentence. Here, the first element is Der Nachbar, and the second is spricht, so the sentence correctly starts Der Nachbar (subject) spricht (verb).

What does gern mean, and why is it used here?

gern means “gladly” or “with pleasure”—in English we might say “likes to.” Placing gern after the verb spricht tells us the neighbor enjoys speaking. You could also write Der Nachbar spricht gerne …, with gerne being a more formal variant, but both forms are common and interchangeable.

Why is it am Nachmittag instead of just Nachmittag or nachmittags?

The preposition an with the dative article dem contracts to am, giving am Nachmittag, which specifies “in the afternoon” as a time frame. If you say nachmittags, you mean “in the afternoons” as a regular habit. Here, am Nachmittag fits because it refers to “that afternoon” or “the afternoon” in general when he has time.

Why is there a comma before wenn er Zeit hat?

In German, subordinate clauses introduced by wenn (“when/if”) are always separated from the main clause by a comma. That comma signals the start of the conditional/time clause wenn er Zeit hat.

Why does hat come at the very end of the subordinate clause?

In German subordinate clauses (those starting with conjunctions like wenn), the conjugated verb moves to the final position. So er Zeit hat places hat at the end.

What is the role of Zeit haben in German?

Zeit haben literally means “to have time.” It’s how you say “to be available” or “to have free time.” In English we might say “when he has time,” and in German the structure is identical: wenn er Zeit hat.

Could we replace wenn with als or wann?

No. wenn is used for repeated or general conditions (“when/if”), which fits here because the neighbor speaks whenever he has time. als would refer to a single past event (“when” once), and wann is used only for direct or indirect questions (“when?”).

Are there any word-order changes if we move am Nachmittag to the front?

Yes. If you start with Am Nachmittag, you still need the verb in second position:
Am Nachmittag spricht der Nachbar gern, wenn er Zeit hat.
Notice spricht stays second, and the subject der Nachbar follows.