In die oop vrieskas ontdooi die vleis en bederf die kos.

Because Afrikaans is a verb-second language in main clauses.

Here, the sentence begins with the place phrase In die oop vrieskas. Once that comes first, the finite verb must come next, so you get:

In die oop vrieskas ontdooi die vleis ...

If you start with the subject instead, you get the more basic order:

Die vleis ontdooi in die oop vrieskas ...

So ontdooi is before die vleis because the sentence does not begin with the subject.

Yes. That is also correct Afrikaans.

The difference is mainly focus:

• Die vleis ontdooi in die oop vrieskas = starts with the subject, so the meat is the starting point.
• In die oop vrieskas ontdooi die vleis = starts with the location, so the place is highlighted first.

Both are grammatical.

Yes. In this sentence, die vleis is understood as the subject of both verbs:

• ontdooi
• bederf

So grammatically, it means:

the meat thaws and spoils the food

Afrikaans often leaves the subject unstated in the second part when it is the same as the first one.

Then you would need to make die kos the subject of the second clause.

A simple version would be:

... en die kos bederf.

That changes the structure from:

• the meat spoils the food → die vleis bederf die kos to
• the food spoils → die kos bederf

So the original sentence does not mean and the food spoils. It means and spoils the food.

Because Afrikaans does not need an auxiliary like English is for the simple present.

English often uses:

• is thawing
• is spoiling

Afrikaans usually just uses the main verb:

• ontdooi
• bederf

So die vleis ontdooi can mean the meat thaws or, depending on context, the meat is thawing.

If you want to make the ongoing action extra explicit, Afrikaans can use a construction like:

Die vleis is besig om te ontdooi.

But that is not required here.

Die is the Afrikaans definite article, meaning the.

Afrikaans uses die for:

• singular nouns
• plural nouns
• all grammatical genders

So you get:

• die oop vrieskas
• die vleis
• die kos

Unlike English learners of languages such as German or French, Afrikaans learners do not have to memorize different forms of the for different genders.

Because Afrikaans usually writes compound nouns as one word.

vrieskas is made from:

• vries = freeze
• kas = cupboard / cabinet / chest

So together: vrieskas = freezer

This one-word spelling is very typical in Afrikaans.

ont- is a very common Afrikaans prefix. It often gives the idea of:

• undoing
• removing
• reversing
• becoming free from something

So ontdooi is literally something like un-freeze, which is why it means thaw or defrost.

For an English speaker, ont- can sometimes feel a bit like:

• un-
• de-
• dis-

depending on the word.

Because in normal Afrikaans, a short fronted phrase like In die oop vrieskas usually does not need a comma.

So this is standard:

In die oop vrieskas ontdooi die vleis ...

A comma might appear only for special stylistic reasons, or if the opening part is long and the writer wants to make the sentence easier to read.

Yes.

Here:

• oop = open
• vrieskas = freezer

So die oop vrieskas means the open freezer.

Like English, Afrikaans normally puts an attributive adjective before the noun:

• oop vrieskas
• koue vleis
• warm kos

So this part of the sentence is very straightforward for an English speaker.