Der Apfel sieht frisch aus.

fresh

Breakdown of Der Apfel sieht frisch aus.

• der Apfel: the apple
• aussehen: to look

Questions & Answers about Der Apfel sieht frisch aus.

What is the structure of the verb aussehen, and why is it written as sieht … aus?

aussehen is a separable verb made up of the root sehen and the prefix aus-. In a main clause the conjugated part (sieht) occupies the second position, and the prefix (aus) moves to the end of the clause:
Der Apfel | sieht | frisch | aus.

Why is Der Apfel in the nominative case?

Because Der Apfel is the subject of the sentence. The verb aussehen does not take an object; it links a subject (in nominative) to a predicative adjective.

Why does frisch have no ending like frischer or frisches?

When an adjective is used predicatively (i.e. after a verb like aussehen), it remains in its base form and is not declined.

Could I replace frisch with another adjective, for example saftig?

Yes. Any adjective used predicatively stays uninflected. For example:
Der Apfel sieht saftig aus.

How would I form a yes/no question from this sentence?