Translation:I aim to have three children.
The raw infinitive of the verb form would be streven naar, but the preposition naar needs an object.
Even though it technically is the primary object of the sentence, the entire subclause "om drie kinderen te hebben" cannot play the role of the object for naar.
Therefore a provisional "het" (it) is added -- which turns into one of the various uses of the word er -- as the object for the preposition naar -- which then finally becomes the pronominal adverb ernaar.
Streven naar = to aim/strive (for)
An object is necessary to accompany the 'loose' preposition naar -- thus one of the numerous roles for the word er is utilised. This then creates the pronominal adverb ernaar, which is present in the sentence:
- Ik streef ernaar om drie kinderen te hebben.
You can aim to do something in the future without actually trying to do it in the present. So you might aim to have three children eventually, but not be trying to do it right now because you're in school, or unemployed, or whatever. So I think you need to use aim (or strive) as opposed to try to.
Clarified with my (Flemish) wife that this is largely a difference in tone. Therefore, "I want" should be a valid translation for "Ik streef ernaar", as that is what we would normally say ("I want to graduate next year" is the same as "I aim to graduate next year"), but I would not use the reverse translation ("ik streef ernaar" when you want to say "I want") because it would end up sounding silly if you said something like "I strive to have eggs for breakfast."
I don't think that should be accepted, due to the same problem that msmith1047 brought up for want. I think for intend it's even worse.
With want the problem is healed to some extent by the fact it's the most natural choice of word. But intend isn't. If you strive to have three children, it means you have been building your entire life around that goal for years. If you intend to have three children it just means you are currently planning to have them. It's quite possible that you will change your mind tomorrow because you are only intending to have them as that's what your partner wants.
Ernaar streven om te [infinitive] is Dutch for strive for [gerund]. There are several ways of answering your question. Just pick one that makes it easy for you:
- Om the is part of the usage pattern for streven. Just learn it along with the verb. It's the same for many other Dutch verbs, though some use just plain te rather than om te.
- Dutch chooses between plain te (to) and om te (for to) based on whether it is remotely reasonable to read te as in order to. (This is similar to how German selects between plain zu (to) and um zu (for to), but Dutch is much more generous in its use of om. In particular, German uses plain zu in connection with danach streben (to strive).)
- Think of om as the Dutch translation of for, and of te (to) as part of the infinitive, which Dutch uses instead of the gerund. (The problem with this explanation is that it will probably make you predict the behaviour of some other verbs incorrectly.)