Because Afrikaans uses is for the present tense of to be with all subjects.
Unlike English, Afrikaans does not say:
with different present-tense forms. Instead, it uses is across the board:
So Die piesangs is goedkoop is completely normal.
Vandag means today, and here it functions as a time adverb.
It tells you when the bananas are cheap:
In Afrikaans, adverbs like vandag often come after the verb, so this word order is very natural.
Because goedkoop comes after the verb is, where it works as a predicate adjective.
In Afrikaans:
Compare:
So in your sentence, goedkoop stays in its basic form.
Here, daar is the existential there, as in English there is or there are.
So:
It does not mean a physical place here. It is just used to introduce the existence of something.
In the sentence omdat daar 'n groot afslag is, it means because there is a big discount.
In Afrikaans, the indefinite article is 'n, which corresponds to English a or an.
A few important points:
So:
This is one of the most distinctive spelling features in Afrikaans.
Because Afrikaans adjectives do not always take an extra ending.
Before a noun, many adjectives do take -e, but some common adjectives can appear without change in certain set patterns or depending on how the language naturally uses them. Groot afslag is the normal, natural combination here.
You will often learn adjective patterns gradually through exposure, because Afrikaans adjective inflection is simpler than in some languages, but not completely automatic in every case.
For this sentence, the important thing to remember is that 'n groot afslag is the correct form.