Learn German: Hat er dir geschrieben, ob das Ergebnis gut ist? - Has he written to you whether the result is good?

The word ob (whether/if) introduces a subordinate clause. In German, subordinate clauses always push the conjugated verb (in this case, ist) to the very end of that clause.

Can I use wenn instead of ob to say "if the result is good"?

No, you have to use ob here. In English, we often use "if" to mean "whether", but German distinguishes between the two. Use ob for indirect yes/no questions where you could add "or not" to the end. Use wenn for conditions, like "if it rains, I will stay inside."

Why does the sentence use dir instead of dich?

The verb schreiben (to write) takes a dative object for the person receiving the message. You can think of it as "has he written to you". Therefore, the accusative dich becomes the dative dir.

Why is geschrieben in the middle of the sentence instead of the very end?

In a main clause, the past participle (geschrieben) goes to the end. But here, the main clause is just Hat er dir geschrieben. The comma signals the start of a completely new clause (the indirect question), so geschrieben sits at the end of its own main clause.