Learn German: Sooft der Anrufer spricht, ignoriere ich ihn. - Whenever the caller speaks, I ignore him.

Both mean "whenever" or "every time that" and are often interchangeable. However, sooft puts a slightly stronger emphasis on the frequency of the action (literally "as often as"), while immer wenn is the standard, everyday way to say "whenever".

Can I write sooft as two words (so oft)?

When used as a subordinating conjunction to connect two clauses like in this sentence, it must be written as one word: sooft. If you are using it as an adverbial phrase simply meaning "so often" (for example, "Why do you call so often?" -> Warum rufst du so oft an?), it is written as two separate words.

Why does the main clause start with the verb ignoriere?

This is a quick refresher on the V2 (verb-second) rule. The entire subordinate clause (Sooft der Anrufer spricht) occupies the first position of the overall sentence. To keep the main verb in the second position, ignoriere must come immediately after the comma, followed by the subject (ich).

Why use ihn at the end of the sentence?

The word ihn is the masculine accusative pronoun for "him". It refers back to der Anrufer (the caller). Because the caller is the direct object receiving the action of the verb ignorieren (to ignore), the pronoun must take the accusative case.