Learn Hardcore German: Nach jeder Frage setze ich ein Fragezeichen. - After every question I put a question mark.

German main clauses follow the “verb-second” (V2) rule. If you put any element other than the subject (here, the adverbial phrase Nach jeder Frage) in first position, the finite verb must stay in second position. That forces the subject ich to move after the verb.

What case does Frage take after the preposition nach, and why is it jeder instead of jede?

Nach always governs the dative. Frage is a feminine noun, so in singular dative you use the ending -er on the indefinite form: nach jeder Frage.

Why is the verb setzen used here? Does it mean the same as English “to set”?

In this context, setzen means “to place” or “to put.” It’s not a literal calque of English “to set,” but both languages share similar uses: you “place” a question mark at the end of a question.

Why is there an indefinite article ein before Fragezeichen? Could we leave it out?

German requires an article before singular countable nouns. Ein Fragezeichen is indefinite because you’re referring to “a question mark” in general. You cannot omit the article here.

What is the structure of the compound noun Fragezeichen?

It’s built from two nouns: Frage (“question”) + Zeichen (“sign” or “symbol”). German often forms compound nouns by sticking words together.

Why does ich come after setze? Isn’t the normal order subject–verb–object?

Because of the V2 rule. When you start the sentence with an adverbial phrase (Nach jeder Frage), the finite verb (setze) occupies the second slot, so the subject (ich) follows in the third slot.

Could I rephrase this with a different introductory phrase, for example Jedes Mal, wenn ich eine Frage beende?

Yes. You can say:
Jedes Mal, wenn ich eine Frage beende, setze ich ein Fragezeichen.
It conveys the same idea with a slightly different style.