Because in this sentence kort comes after the verb is, where Afrikaans adjectives usually stay in their basic form.
Compare the pattern:
So:
In standard Afrikaans, negation usually uses nie ... nie.
So:
This means:
A very important Afrikaans rule:
Afrikaans nouns do not work with grammatical gender in the way many European languages do.
So for everyday sentence building, you do not need to learn verklaring as masculine or feminine.
You can simply learn:
That makes Afrikaans simpler than languages that require gender agreement.
A rough English-friendly guide would be something like:
A few pronunciation notes: