This is a very common question.
In Die rem is nuut, the adjective comes after the verb is, so it stays in its basic form: nuut.
But when the adjective comes before the noun, it usually takes -e:
So compare:
This difference between nuut and nuwe is one of the most important adjective patterns in Afrikaans.
Because in a simple statement like this, Afrikaans and English use a very similar pattern:
subject + verb + complement
So:
That gives: Die rem is nuut.
Afrikaans word order becomes more different from English in some other sentence types, but this basic sentence is very straightforward.
Afrikaans usually uses a double negative pattern with nie ... nie.
So:
This is one of the most important features of Afrikaans grammar: