We use _dat_ to refer to het-words:
Het boek dat ik wil kopen, is al weken uitverkocht. | The book that I want to read has been sold out for weeks.
Ik vond het verhaal dat hij aan ons voorlas , maar griezelig. | I thought the story that he read to us was rather creepy.
Het concept , dat door mijn collega is bedacht, is een groot succes geworden. | The concept, which was designed by my colleague, turned out to be a great success.
Het vliegtuig , dat voor elke vlucht streng gecontroleerd wordt, verloor tijdens het opstijgen een wiel. | The airplane, which is scrutinously checked before each flight, lost a wheel during take-off.
Dat - which/that
If you look at the sentences, you can see that the Dutch relative clauses all begin with 'dat', while the English clauses start with either 'that' or 'which'.
It is a distinction between two types of relative clauses.
The first type gives us extra information. If we omit the relative clause, the sentence will still make sense. For this type of relative clause, the English use 'which'. The Dutch only indicate the 'redundancy' of the extra information by adding an extra comma before the clause.
The second type is a necessary part of the sentence. In English, this clause always begins with 'that'. We do not add a comma before this clause (neither in Dutch, nor in English). The Dutch do add a comma at the end of the clause.
_Dat_ turning into _waar_
If _dat_ is preceded by a preposition, it turns into a [pronominal adverb](#/lesson/Pronouns.Ad02). This means that we replace _dat_ by _waar_ and we glue the preposition to the end of it.
preposition + dat | | waar +preposition
Do not confuse this with the [demonstrative pronoun _dat_](#/lesson/Pronouns.De04), which is replaced by _daar_.
As you can read in [the Dutch pronominal adverb](#/lesson/Pronouns.Ad02), the pronominal adverb is often separated by other verbs.
Het boek waar ik naar op zoek ben.
Not: Het boek ~~naar dat~~ ik op zoek ben. | The book that I am looking for.
["the book wherefor I am looking"]
Het verhaal waar we nachtmerries van kregen.
Not: Het verhaal ~~van dat~~ we nachtmerries kregen. | The story that gave us nightmares.
["The story whereof we got nightmares"]
Het concept waar mijn collega beroemd mee is geworden.
Not: Het concept ~~met dat~~ mijn collega beroemd is geworden. | The concept that made my colleague famous.
["The concept wherewith my colleague became famous"]
Het vliegtuig waar we in zaten.
Not: Het vliegtuig ~~in dat~~ we zaten. | The airplane that we were in.
["The airplane wherein we were"]
While most prepositions do not change when they become part of a pronominal adverb, there are two exceptions: _Met_ and _tot_. They turn into _mee_ and _toe_ respectively.