Onderstelnommer is a compound noun, which is very common in Afrikaans.
It is made up of:
Together:
Afrikaans often combines nouns into one word where English may use two words or a phrase. This is something learners see a lot.
Examples:
So the long word is not unusual—it is just a compound.
Because Afrikaans usually writes compound nouns as a single word.
English often leaves these as separate words, but Afrikaans joins them:
If two nouns belong together as one idea, Afrikaans often combines them into one written word.
Is is the present-tense form of wees for this kind of sentence, just like English is from to be.
Here it links the subject to a location:
A useful thing for learners: in the present tense, Afrikaans does not change is according to person the way some languages do.
For example:
So is is very common and very useful.
Here die is usually pronounced roughly like dee.
So:
This is different from the English word die.
That difference is important, because English speakers often want to pronounce it like the English verb die, but in Afrikaans that would be incorrect here.
The main things are:
So although the sentence may look unfamiliar at first because of the long compound noun, its grammar is actually quite simple.