Most of the time the relative pronoun for a het-word is dat — het boek dat ik lees. But in a specific, well-defined set of cases Dutch switches to wat. You use wat after the indefinite neuter words alles, iets, niets, veel, weinig; after the demonstrative dat used as a noun; after a neuter superlative (het mooiste wat...); and — most importantly — when the relative clause refers back to a whole preceding clause rather than to a single noun. The unifying idea is that wat points to something vague, abstract, or sentence-sized, whereas dat points to a specific, countable het-noun. English uses that and which for all of these, so the dat-vs-wat split is a distinction English speakers must learn from scratch. This page draws the line precisely.
After alles, iets, niets, veel, weinig
These indefinite quantity words are grammatically neuter but they do not name a concrete thing — they are open-ended. When a relative clause modifies them, the pronoun is wat, never dat.
Alles wat hij zegt, klopt.
Everything he says is right. 'alles' → 'wat', never 'alles dat'.
Er is niets wat me nog kan verbazen.
There's nothing that can still surprise me. 'niets' → 'wat'.
Is er iets wat ik voor je kan doen?
Is there something I can do for you? 'iets' → 'wat'.
Er is veel wat ik nog niet begrijp.
There's a lot I still don't understand. 'veel' → 'wat'.
After the demonstrative dat ("that which")
When the neuter demonstrative dat ("that thing") is itself the antecedent, the relative pronoun is again wat — giving the pattern dat wat ("that which / what"). This avoids stammering dat dat and matches the abstract, pointing sense.
Dat wat hij gisteren zei, was niet aardig.
What he said yesterday wasn't nice. 'dat wat' = 'that which'; the antecedent is the demonstrative 'dat'.
Geloof niet alles, maar dat wat je zelf ziet.
Don't believe everything, but that which you see for yourself. 'dat wat' again.
In everyday speech this dat wat often collapses to a bare wat ("what"), functioning as a free relative: Wat hij zei, was niet aardig. That is the same construction with the demonstrative dropped.
After a neuter superlative
A superlative adjective used as a neuter noun — het mooiste (the most beautiful), het beste (the best), het ergste (the worst) — takes wat. The superlative is treated like an indefinite abstract quantity, not a concrete het-noun.
Dit is het mooiste wat ik ooit heb gezien.
This is the most beautiful thing I've ever seen. 'het mooiste' (superlative) → 'wat', cluster 'heb gezien' at the end.
Het beste wat je kunt doen, is gewoon wachten.
The best thing you can do is simply wait. 'het beste' → 'wat'.
Dat was het ergste wat ons had kunnen overkomen.
That was the worst thing that could have happened to us. 'het ergste' → 'wat'.
Contrast this with a superlative that modifies a concrete het-noun: het mooiste schilderij dat ik ken (the most beautiful painting I know) takes dat, because the antecedent is now the specific noun schilderij, not the bare superlative. The presence of a real noun flips you back to dat.
When the antecedent is a whole clause
This is the most frequent and most useful case. When the relative clause comments on the entire preceding statement — not on any single noun — the pronoun is wat ("which"). English uses which here too, often after a comma: He arrived late, which annoyed me.
Hij kwam te laat, wat ik vervelend vond.
He arrived late, which I found annoying. 'wat' refers to the whole idea 'he arrived late'.
Ze heeft de baan gekregen, wat we allemaal hadden verwacht.
She got the job, which we'd all expected. 'wat' = the whole event, not a noun.
Het regende de hele dag, wat onze plannen verpestte.
It rained all day, which ruined our plans. 'wat' points back to the entire clause.
A useful test: if you could replace the relative pronoun with "which (fact/thing)" referring to the situation rather than a noun, you need wat. A comma almost always precedes this clausal wat.
The boundary: a specific het-noun keeps dat
The error to avoid is over-extending wat to ordinary, concrete het-words. A specific het-noun — het boek, het huis, het kind, het idee — takes dat, never wat. Wat is for the vague and the sentence-sized; dat is for the countable and concrete.
Het idee dat je voorstelde, is briljant.
The idea you proposed is brilliant. 'het idee' is a specific het-noun → 'dat', not 'wat'.
Het boek dat op tafel ligt, is van mij.
The book on the table is mine. Specific het-noun → 'dat'.
Common Mistakes
❌ Alles dat hij zegt, is waar.
Incorrect — 'alles' always triggers 'wat', not 'dat'.
✅ Alles wat hij zegt, is waar.
Everything he says is true.
❌ Er is niets dat me kan stoppen.
Incorrect — 'niets' takes 'wat': 'niets wat me kan stoppen'.
✅ Er is niets wat me kan stoppen.
There's nothing that can stop me.
❌ Dit is het mooiste dat ik ooit heb gezien.
Incorrect — a bare neuter superlative ('het mooiste') takes 'wat', not 'dat'.
✅ Dit is het mooiste wat ik ooit heb gezien.
This is the most beautiful thing I've ever seen.
❌ Hij kwam te laat, dat ik vervelend vond.
Incorrect — when the antecedent is a whole clause, use 'wat', not 'dat'.
✅ Hij kwam te laat, wat ik vervelend vond.
He arrived late, which I found annoying.
❌ Het idee wat je voorstelde, is goed.
Incorrect — a specific het-noun ('het idee') takes 'dat', not 'wat'.
✅ Het idee dat je voorstelde, is goed.
The idea you proposed is good.
Key Takeaways
- Use wat after the indefinite neuters alles, iets, niets, veel, weinig, after the demonstrative dat (dat wat), after a bare neuter superlative (het mooiste wat...), and when the antecedent is an entire clause (..., wat...).
- Use dat for a specific, concrete het-noun (het boek dat..., het idee dat...).
- A superlative + a real noun resets you to dat: het mooiste schilderij dat....
- The clausal wat ("which") is almost always preceded by a comma and refers to the whole preceding statement.
- The deep logic: wat = vague / abstract / sentence-sized; dat = countable / concrete het-noun.
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