Learn Hardcore German: Wenn der Pilot, der sehr freundlich ist, fliegt, fühle ich mich sicher. - When the pilot, who is very friendly, flies, I feel safe.

In German, all relative clauses—clauses that give extra information about a noun—are set off by commas. Here, der sehr freundlich ist is a relative clause modifying der Pilot. That’s why you see a comma before der and after ist.

What does the relative pronoun der refer to, and why is it also der?

The pronoun der points back to der Pilot. German relative pronouns agree in gender and number with their antecedent (here: masculine singular), but their case is determined by their role inside the relative clause. Since the pilot is the subject of ist freundlich, we use the nominative form der.

Why does fliegt come at the end of the clause Wenn der Pilot, der sehr freundlich ist, fliegt?

Because wenn introduces a subordinate clause (a “when” clause). In German subordinate clauses, the conjugated verb moves to the end. That’s why fliegt appears after everything else in that clause.

Why is the main clause fühle ich mich sicher and not ich fühle mich sicher?

Since the sentence starts with a subordinate clause (Wenn … fliegt), the very next element in the main clause must be the finite verb (this is called inversion). So you get fühle ich instead of ich fühle. Also, sich fühlen is a reflexive construction, so you need mich (accusative) to say “I feel safe.”

What’s the difference between der sehr freundliche Pilot and der Pilot, der sehr freundlich ist?

Der sehr freundliche Pilot uses an attributive adjective (it directly modifies Pilot, requiring the ending -e on freundlich).
Der Pilot, der sehr freundlich ist uses a relative clause with a predicative adjective (no ending on freundlich because it follows ist).
Both mean “the very friendly pilot,” but the attributive form is more compact, while the relative clause can feel more conversational or emphatic.

Could you drop ist and say der Pilot, der sehr freundlich fliegt?

No. Fliegen is an action verb (“to fly”), while freundlich is an adjective (“friendly”). Der Pilot, der sehr freundlich fliegt would mean “the pilot who flies very friendly,” which is grammatically odd and changes the meaning. To describe the pilot’s personality, you need ist plus the adjective.

Can I move the relative clause so that it comes after the main clause, like Wenn der Pilot fliegt, der sehr freundlich ist, fühle ich mich sicher?

Technically you can, but it’s very unusual. In German, relative clauses are normally placed immediately after their noun to keep the sentence clear. Splitting der Pilot and its clause der sehr freundlich ist by fliegt makes the sentence harder to process.