1. Personal pronouns are used after prepositions when referring to people. However,
when you need to refer to a thing, a compound using er- plus the preposition
(either written as one word, or separated by adverbial expression)
is used. Daar (that) and hier (this) can also replace er when it
is not written as one word.

De kinderen spelen vaak ermee.

The children often play with it.
De kinderen spelen er vaak mee. The children often play with it.
Di kinderen spelen daar/hier vaak mee. The children often play with
that/this.

2. Er is used when talking about a quantity or an
amount. It is translated as "of it" or "of them," though these expressions
are not always used in English.

Ik heb er genoeg gehad. I've had enough (of it.)
Hoeveel poesjes heb je? Ik heb er twee. How many kittens do
you have? I have two (of them.)

3. In an unstressed position,
er means there (an adverb of place). It is replaced by daar

in stressed positions (such as the beginning of a sentence.)

4. Er
can introduce sentences with an indefinite subject. In this case, er
functions as there as a subject, as in "there is/are."

This article was used with permission from:
Indo-European Languages