Ek was moeg, en toe het ek gaan slaap.

Because was is the past tense of is / wees.

• Ek is moeg = I am tired
• Ek was moeg = I was tired

In this sentence, the speaker is talking about a past situation, so was is the right form.

Afrikaans often uses was as the normal past form of wees in simple statements like this.

So:

• Ek was moeg = the normal, natural way to say I was tired
• Ek het moeg gewees is also possible, but it is less simple and less natural in a sentence like this

A good learner rule is:

• for to be in the past, was is very common
• for many other verbs, Afrikaans usually uses het
  • another verb form

Here, toe means then or so then.

So:

• en toe = and then

It shows the next event in the story:

1. Ek was moeg
2. toe het ek gaan slaap

So the tiredness comes first, and sleeping comes after that.

Because this is a main clause, and Afrikaans usually puts the finite verb in the second position.

In toe het ek gaan slaap:

• toe is first
• het must come second
• ek comes after that

So the pattern is:

• toe + het + ek + ...

This is very common in Afrikaans. If something other than the subject comes first, the verb usually moves into second position.

Compare:

• Ek het gaan slaap = I went to sleep
• Toe het ek gaan slaap = Then I went to sleep

Het is the auxiliary verb used to form the past for many Afrikaans verbs.

So in the second clause, het helps show that the action happened in the past:

• het ek gaan slaap

Even though English translates this as I went to sleep, Afrikaans often builds past actions with het.

So do not try to translate het word-for-word every time. Here it is just part of the past-tense structure.

Because gaan slaap means go to sleep, not just sleep.

There is a difference:

• Ek het geslaap = I slept
• Ek het gaan slaap = I went to sleep

So gaan slaap is a verb combination that expresses the idea of going off to sleep or going to bed to sleep.

In this sentence, the meaning is not simply that the person slept, but that they went to sleep after being tired.

It is two words, but together they act like one verbal idea.

• gaan = go
• slaap = sleep

Together, gaan slaap means go to sleep.

So grammatically it is a verb combination, but for meaning, it is often easiest to learn it as a chunk.

Yes. You could say:

• Ek was moeg en ek het gaan slaap.

That is still correct.

But toe adds the sense of then, after that, or so. It makes the sequence clearer and sounds very natural in storytelling and conversation.

So:

• Ek was moeg en ek het gaan slaap = I was tired and I went to sleep
• Ek was moeg, en toe het ek gaan slaap = I was tired, and then I went to sleep

Because the second part is a full clause of its own, and Afrikaans normally states the subject again.

So:

• Ek was moeg
• en toe het ek gaan slaap

Even though the subject is the same person, Afrikaans still uses ek again, just like English does in I was tired, and then I went to sleep.

Yes, but not in this sentence.

In your sentence, toe means then because it is followed by main-clause word order:

• toe het ek gaan slaap

But toe can also mean when in a different structure:

• Toe ek moeg was, het ek gaan slaap.
• When I was tired, I went to sleep.

Notice the difference:

• toe het ek ... = then I ...
• toe ek ... was = when I was ...

So the meaning of toe depends a lot on the sentence structure.