Ek is jammer dat ek jou nie gister teruggebel het nie.

It means I am sorry.

A few useful notes:

• jammer means sorry, regretful, or sad about something
• Ek is jammer is a very common way to apologize or express regret
• In this sentence, it is followed by a dat-clause explaining what the speaker is sorry about

So:

• Ek is jammer = I’m sorry
• Ek is jammer dat... = I’m sorry that...

dat means that.

It introduces the clause that explains the reason for the regret:

• Ek is jammer = I’m sorry
• dat ek jou nie gister teruggebel het nie = that I didn’t call you back yesterday

In English, that is often optional:

• I’m sorry that I didn’t call you back
• I’m sorry I didn’t call you back

In Afrikaans, dat is very normal and standard in this kind of sentence.

Because dat ek jou nie gister teruggebel het nie is a full clause of its own, and that clause needs its own subject.

So the structure is:

• Ek is jammer = main clause
• dat ek ... het nie = subordinate clause

That second ek means I, the subject of called back.

English works the same way:

• I am sorry that I didn’t call you back yesterday

This is one of the most important features of standard Afrikaans: double negation.

In many negative sentences, Afrikaans uses:

• a first nie
• and then a second nie near the end of the clause

So here:

• dat ek jou nie gister teruggebel het nie

This means that I did not call you back yesterday.

Very roughly:

• first nie introduces the negation
• final nie closes the negative clause

English only needs one negative word, but standard Afrikaans usually needs both.

Because dat introduces a subordinate clause, and subordinate clauses in Afrikaans have different word order from main clauses.

Compare:

• Main clause: Ek het jou nie gister teruggebel nie.
• After dat: dat ek jou nie gister teruggebel het nie

Notice that in the dat-clause, the finite auxiliary het moves toward the end.

A simple way to think about it:

• in a main clause, the verb comes early
• in a subordinate clause, the verb part often comes later or at the end

Because Afrikaans often uses the perfect tense where English uses the simple past.

So:

• het teruggebel is literally like have called back
• but in normal English, with yesterday, we say called back or did call back

That means:

• Ek het jou gister teruggebel = I called you back yesterday

Even though Afrikaans uses het, the natural English translation is usually simple past.

The verb is terugbel, meaning to call back.

This is a compound verb, and in the past participle, ge- goes between the first part and the verb stem:

• terug + ge + bel = teruggebel

So:

• terugbel = to call back
• teruggebel = called back

Compare:

• opbel → opgebel
• terugkom → teruggekom

Also note that in some sentence patterns, the parts can appear separated:

• Ek bel jou later terug. = I’ll call you back later.

But in the participle, it is written as one word: teruggebel.

Because this is a subordinate clause after dat.

In a main clause, you would normally say:

• Ek het jou nie gister teruggebel nie.

But after dat, the auxiliary het moves to the end:

• dat ek jou nie gister teruggebel het nie

So the difference is mainly about clause type:

• main clause: het comes earlier
• subordinate clause: het comes later, often at the end

jou is the object form of jy, meaning you.

Here it means the person being called back:

• ek = I
• jou = you

So:

• ek jou teruggebel het = I called you back

A useful comparison:

• jy = you, as subject
• jou = you, as object

Examples:

• Jy bel my. = You call me.
• Ek bel jou. = I call you.

Here, jou is the normal informal singular you.

Compare:

• jy / jou = informal singular you
• julle = plural you
• u = formal you

So if you are speaking to one friend, jou is exactly what you would expect here.

gister means yesterday, and it is a time adverb.

In this sentence, it comes before the final verb group:

• dat ek jou nie gister teruggebel het nie

That is a very natural Afrikaans order.

A rough pattern here is:

• subject
• object
• nie
• time word
• participle
• auxiliary
• final nie

There is some flexibility in Afrikaans word order, but this version sounds normal and idiomatic.

Not necessarily.

In this sentence, the first nie is part of the normal Afrikaans negative structure, so the whole clause is negative:

• I didn’t call you back yesterday

It does not automatically mean a strong contrast like:

• not yesterday, but today

If you wanted that kind of contrast, you would usually show it through stress or extra wording.

So the sentence is normally understood simply as an apology for not calling back yesterday.

Yes, especially in speech.

You can say:

• Ek is jammer ek het jou nie gister teruggebel nie.

That sounds natural in everyday Afrikaans.

But the version with dat:

• Ek is jammer dat ek jou nie gister teruggebel het nie.

is also completely natural and a bit more explicit, especially in careful speech or writing.