“Co” or “což”? When to use each

Here’s the whole idea in one line: use co when it points back to one noun, and což when it comments on a whole preceding idea. The ending -ž is the signal that you’re reacting to the entire situation.

1. “Co” = “který” (points to a noun)

In everyday Czech, co often replaces the relative pronoun který / které (“which / that”). It refers to a specific thing you just named.

The test: if you can swap the word for který / které and the sentence still works, use plain co.

2. “Což” = “and that whole thing…”

Use což when “which” refers not to a single noun but to everything you just said — a whole clause, a whole situation. Here který is impossible.

3. “Což” changes case: čemuž, čehož

Just like any pronoun, což takes whatever case the verb after it demands. Two you’ll meet a lot:

věřit + dative → čemuž (“which” as the thing believed):

litovat + genitive → čehož:

4. Bonus trap: “čemu” is not “proč”

Some verbs simply take a case, and the question word changes with them. Rozumět (“to understand”) takes the dative, so you ask with čemu — “to what,” not “why”:

The near-synonym chápat (“to grasp / get”) takes the accusative, so there you use co:

Remember the -ž signal: it means “I’m commenting on the whole thing.” Everything else is plain co / čemu — the same words you’d use in a question.