Die mieliepap, slaai en sous is bykosse.

Die is the Afrikaans definite article, so it corresponds to English the. Unlike English, Afrikaans uses die for both singular and plural nouns.

So you can have:

• die sous = the sauce
• die bykosse = the side dishes

In this sentence, die marks the noun phrase as definite.

Here, die can be understood as introducing the whole list: mieliepap, slaai en sous.

So the sense is:

• the mieliepap, salad, and sauce

Afrikaans often does this with coordinated lists. If you wanted to be more explicit, you could also repeat the article:

• Die mieliepap, die slaai en die sous ...

But repeating die is not necessary here.

Because each word names one item or one type of food in the list.

This works much like English:

• the salad and sauce are ...

You do not need plural forms on each noun just because there is more than one thing in the overall subject. The subject is plural because it is a combination of several singular nouns joined by en.

In present-tense Afrikaans, is is used for both singular and plural subjects. Afrikaans does not change the present form of to be the way English does.

Compare:

• Die slaai is vars. = The salad is fresh.
• Die slaai en sous is vars. = The salad and sauce are fresh.

So is can mean is or are, depending on the subject.

En means and. It joins the items in the list:

• mieliepap, slaai en sous = mieliepap, salad and sauce

This is the normal way to connect nouns in Afrikaans.

The singular is bykos and the plural is bykosse.

In this sentence, the three foods together are being classified as more than one side dish, so the plural is needed.

Afrikaans plural formation varies by noun. For bykos, the plural is formed with -se:

• bykos → bykosse

That is just the standard plural form for this noun.

Because bykosse is being used as a predicate noun after is. In Afrikaans, as in English, when you say what something is, you often leave out the article.

Compare the English pattern:

• They are side dishes. not usually
• They are the side dishes.

So:

• ... is bykosse = they belong to the category of side dishes

If you said die bykosse, that would usually sound more specific, as if you meant the side dishes in a particular set.

Afrikaans usually writes compound nouns as one word. Mieliepap is a compound noun made from:

• mielie = maize/corn
• pap = porridge

So Afrikaans prefers mieliepap rather than writing it as two separate words. This is very common in the language.

Mostly yes. The sentence lists several items separated by commas, with en before the last one.

Notice that there is no comma before en:

• Die mieliepap, slaai en sous ...

That is normal Afrikaans punctuation. In English, some writers use an Oxford comma before and, but in Afrikaans the version without that extra comma is the usual choice.

You could, but it would mean something more specific.

• ... is bykosse = they are side dishes in general
• ... is die bykosse = they are the side dishes, meaning a specific known set

So the original sentence is the more natural choice if you are simply identifying the foods by category.