Because it is being used after the verb is as a predicate adjective.
In Afrikaans, adjectives used this way usually stay in their basic form:
So droog does not need an ending here.
Usually plastiek means plastic as a material.
In Die plastiek is droog, it most naturally refers to a specific piece, amount, or type of plastic that both speaker and listener know about.
If you wanted to talk about plastic in general, Afrikaans can also say:
That would sound more general, while Die plastiek sounds more specific.
Sometimes, yes, but it changes the meaning slightly.
So whether you include die depends on whether you mean the specific plastic or plastic in general.
Afrikaans normally uses a double nie structure for negation:
Die plastiek is nie droog nie.
That means The plastic is not dry.
A simple way to see it is:
A rough guide is:
A few helpful notes:
So the whole sentence is roughly:
dee plahs-TEEK is drohkh