Learn Hardcore German: Für das Interview trug er eine dunkle Sonnenbrille, weil der Akku seiner Kamera leer war und er draußen warten musste. - For the interview he wore dark sunglasses because the battery of his camera was empty and he had to wait outside.

The preposition für always governs the accusative case. Since Interview is a neuter noun, its definite article in the accusative is das, giving für das Interview (“for the interview”).

Why is the verb in the main clause trug instead of hat getragen or a present-tense form?

German has two common past tenses: the simple past (Präteritum) and the perfect (Perfekt). In written or formal narrative style, the Präteritum (here trug, from tragen) is preferred. In everyday spoken German you’d often hear hat getragen instead.

What gender and case is eine dunkle Sonnenbrille, and why is the adjective ending -e?

Sonnenbrille is feminine, and it acts as the direct object of trug, so it’s in the accusative case. The indefinite article for accusative feminine is eine, and after an article that shows the case, the attributive adjective takes the weak ending -e, hence eine dunkle Sonnenbrille.

Why is there a comma before weil?

Weil introduces a subordinate (dependent) clause in German. Standard punctuation requires you to separate an independent clause from a subordinate clause with a comma.

Why does the verb war appear at the end of its clause: … weil der Akku seiner Kamera leer war?

Subordinate clauses in German follow verb-final word order. Any subordinating conjunction (like weil) pushes the conjugated verb (war) all the way to the end of that clause.

What case is seiner Kamera, and why isn’t it just seine Kamera?

Seiner Kamera is the genitive form showing possession (“of his camera”). The possessive pronoun sein declines like an article; in the genitive singular for a feminine noun you use seiner, so der Akku seiner Kamera means “the battery of his camera.”

Why does the subordinate clause continue with und er draußen warten musste instead of repeating weil?

You can coordinate two subordinate clauses under a single weil by linking them with und. Both clauses remain subordinate with verb-final word order:
• der Akku … leer war
• er draußen warten musste

Why is draußen placed before the verb cluster warten musste?

In a subordinate clause, adverbials (like draußen, indicating place) can appear early for emphasis or clarity, while all verbal parts (here the infinitive warten and the modal musste) form a block at the very end.

Could this sentence be rephrased to start with the weil-clause?

Yes. For example:
Weil der Akku seiner Kamera leer war und er draußen warten musste, trug er eine dunkle Sonnenbrille für das Interview.
When a subordinate clause comes first, it still ends with its verb(s), and you place a comma before the main clause, which then has normal second-position verb order.

Could we use denn instead of weil, and what’s the difference?

Denn is a coordinating conjunction expressing “for” or “because,” but it does not send the verb to the end and does not create a true subordinate clause. With denn you’d keep the normal SVO word order and typically use a comma or none at all:
Für das Interview trug er eine dunkle Sonnenbrille, denn der Akku seiner Kamera war leer und er musste draußen warten.
In more formal or written German, weil + verb-final is stylistically preferred to signal a subordinate causal clause.