An old-fashioned English way of saying "with that" or "for this" is "therewith" or "herefore". We call this a [pronominal adverb](#/lesson/Pronouns.Ad01).
In Dutch, this old-fashioned form is compulsory.
This is what happens:
preposition + dit/deze | | hier +preposition
preposition + dat/die | | daar +preposition
Note that this only applies to [independent demonstrative](#/lesson/Pronouns.De03) pronouns. A pronoun that precedes a noun (e.g. 'this book', 'that day ) is not independent. The examples below will illustrate the idea.
Recall that we use _dit_ and _dat_ for _het_-nouns, _deze_ and _die_ for _de_-nouns (including all plural nouns).
Examples of _dit_ and _deze_ turning into _hier_
_Deze_ (singular) -> _hier_
Ik heb van deze ervaring geleerd. | I have learned from this experience.
Ik heb hiervan geleerd.
Not: Ik heb ~~van deze~~ geleerd. | I have learned from this.
_Deze_ (plural) -> _hier_
Ze schreef over deze gebeurtenissen. | She wrote about these events.
Ze schreef hierover.
Not: Ze schreef ~~over deze~~. | She wrote about these.
_Dit_ (always singular) -> _hier_
Ze werden door dit nieuws verrast. | They were surprised by this news.
Ze werden hierdoor verrast.
Not: Ze werden ~~door dit~~ verrast. | They were surprised by this.
Examples of _dat_ and _die_ turning into _daar_
_Die_ (singular) -> _daar_
Hij ergerde zich aan die stank. | He was annoyed by that smell.
Hij ergerde zich daaraan.
Not: Hij ergerde zich ~~aan die~~. | He was annoyed by that.
_Die_ (plural) -> _daar_
Ze zaten achter die gordijnen. | They were behind those curtains.
Ze zaten daarachter.
Not: Ze zaten ~~achter die~~. | They were behind those.
_Dat_ (always singular) -> _daar_
Ze praten _over dat verhaal_'. | They are talking about that story.
Ze praten daarover.
Not: Ze praten ~~over dat~~. | They are talking about that.