wenn vs. wann. Two words, one English word.

English "when" does two completely different jobs: it asks a question, and it links two statements. German never lets one word do both. wann asks. wenn links. Mixing them up is one of the most common mistakes English-speaking learners make, often without noticing.

One English word, two German words

Look at these two English sentences. Both use "when." In German, they need two completely different words.

English	German	Why
When does the course start?	Wann beginnt derKurs?	It's a question.
When the course starts, I'll be there.	Wenn derKurs beginnt, bin ich da.	It links two clauses.
When are you free?	Wann hast du Zeit?	It's a question.
When you have time, call me.	Wenn du Zeithast, ruf mich an.	It links two clauses.

wann wants an answer: a time, a date, a clock reading. wenn wants nothing. It just connects a condition or a moment to a result.

The trap: indirect questions

The mistake almost never happens with sentences that end in a question mark. It happens with sentences that look like statements but secretly contain a question.

This sentence has no question mark, so it's tempting to reach for wenn. But pull the clause out on its own: "When is he coming?" That's a question. And every hidden question about time belongs to wann, no matter how deep inside the sentence it sits.

English (looks like a statement)	Hidden question	German
I don't know when the train leaves.	When does the train leave?	Ich weiß nicht, wann derZug abfährt.
Can you tell me when the shop opens?	When does the shop open?	Kannst du mir sagen, wann derLaden öffnet?
She asked when we have time.	When do you have time?	Sie hat gefragt, wann wir Zeithaben.

The quick test

Try replacing "when" with "at what time?" If that still makes sense, even as a question buried inside the sentence, use wann. If "when" really means "if," "whenever," or "at the moment that," use wenn.

Sentence with "when"	Replace with "at what time?"	Replace with "if / whenever"	Use
I don't know when she arrives.	✓ I don't know at what time she arrives.	✗	wann
When she arrives, call me.	✗	✓ Whenever she arrives, call me.	wenn
Tell me when the meeting is.	✓ Tell me at what time the meeting is.	✗	wann
When it rains, I stay home.	✗	✓ If/whenever it rains, I stay home.	wenn

Both send the verb to the end, so the order doesn't help

Here's why the confusion is so persistent: both wann and wenn send the verb to the end of their clause when they're not the very first word in the sentence. Word order can't tell them apart, only meaning can.

Clause	Verb	Word	Meaning
...wann derKurs	beginnt	wann	asks "when does it begin?"
...wenn derKurs	beginnt	wenn	states "when/if it begins"

Same word order, same verb position, opposite jobs. The only way through is the question test above.

More on each word individually: wann. Asking for a point in time. and wenn. When or if, the verb goes last. For a single moment in the past, neither applies, that's als. One moment in the past.