Learn Hardcore German: Die Gurken bleiben frisch, wenn ich sie in den Kühlschrank stelle. - The cucumbers stay fresh when I put them in the refrigerator.

Gurke is a feminine noun (die Gurke). In German, all plural nouns—no matter their original gender—take the article die. Since Gurken is the plural form (with the –n ending), it correctly uses die.

How can I tell that Gurken is plural and not singular?

You can spot the plural by three clues:

The noun ends in –n (Gurken).
The article is die, which serves both feminine singular and plural—so you need the ending and verb form to confirm.
The verb is bleiben (plural form) rather than bleibt (singular).

Why is bleiben used instead of bleibt or ist?

bleiben is the plural form of “to remain/stay” and matches the plural subject Gurken.
bleibt would be wrong because that’s the 3rd person singular.
You could say Die Gurken sind frisch (“the cucumbers are fresh”), but bleiben adds the nuance “stay/remain fresh over time.”

Why is the adjective frisch not declined (no ending)?

After linking verbs like bleiben, sein, or werden, adjectives are used predicatively and remain uninflected. You only decline adjectives when they’re attributive (directly modifying a noun), e.g. frische Gurken.

Why is wenn used here instead of wann?

wenn is a subordinating conjunction meaning “if” or “whenever.” It’s used for conditional or repeated‐time clauses. By contrast, wann is only used in questions to ask “when?” (e.g. Wann stellst du…?).

Why is there a comma before wenn?

German grammar requires a comma to separate the main clause from a subordinate clause. That comma signals that wenn introduces a dependent clause.

Why does the verb stelle appear at the end of the clause wenn ich sie in den Kühlschrank stelle?

In German subordinate clauses (those introduced by wenn, dass, weil etc.), the finite verb moves to the end of the clause. Hence stelle comes last.

Why is it in den Kühlschrank (accusative) and not im Kühlschrank (dative)?

in is a two-way preposition (Wechselpräposition).
When you express movement into something (“into the fridge”), you use the accusative.
im (contraction of in dem) is dative and would imply a static location (“in the fridge”).

What does sie refer to, and what case is it?

sie is a plural accusative pronoun referring back to die Gurken. It’s accusative because it’s the direct object of stellen (“I put them in…”).

Can I start with the wenn clause? If so, what changes in the main clause’s word order?

Yes. You can say:
Wenn ich sie in den Kühlschrank stelle, bleiben die Gurken frisch.
Because the subordinate clause comes first, the main clause treats it as the first “element,” so its finite verb (bleiben) appears immediately after the comma, followed by the subject.