Jeg forstår det slet ikke.

A word-by-word breakdown:

• Jeg = I
• forstår = understand / am understanding
• det = it / that
• slet ikke = not at all

So the structure is:

• subject: Jeg
• verb: forstår
• object: det
• negative emphasis: slet ikke

Even if you already know the overall meaning, it helps to see how the sentence is built.

Forstår is the present tense form of the verb at forstå.

• infinitive: at forstå = to understand
• present: forstår = understand / understands

In Danish, the present tense often ends in -r.

Examples:

• jeg forstår = I understand
• du forstår = you understand
• han forstår = he understands

Unlike English, Danish does not change the verb much depending on the subject. The same present-tense form works for all persons.

Slet ikke is a very common Danish expression meaning not at all, absolutely not, or really not depending on context.

In this sentence, it strengthens the negation:

• Jeg forstår det ikke = I don’t understand it
• Jeg forstår det slet ikke = I don’t understand it at all

So slet by itself is usually not something you translate literally here. It mainly works together with ikke to make the negation stronger.

Yes. That is completely correct.

The difference is mainly strength:

• Jeg forstår det ikke = neutral: I don’t understand it
• Jeg forstår det slet ikke = stronger: I really don’t understand it / I don’t understand it at all

So if you want extra emphasis, use slet ikke.

This is a very common word-order point in Danish.

In Danish main clauses, short object pronouns like det, ham, hende, dem often come before ikke.

So:

• Jeg forstår det ikke
• Jeg kender ham ikke
• Jeg kan lide den ikke is not the normal pattern

This may feel different from English, where not usually comes earlier in the clause.

A useful thing to remember:

• pronoun object often comes before ikke
• a full noun often comes after ikke

Compare:

• Jeg forstår det ikke
• Jeg forstår ikke spørgsmålet

Usually, no—not as the normal neutral sentence.

The natural wording is:

• Jeg forstår det ikke

Putting ikke before det can sound marked or contrastive, as if you are stressing that specifically:

• Jeg forstår ikke det, men jeg forstår resten
  = I don’t understand that, but I understand the rest

So for everyday use, stick with Jeg forstår det ikke or Jeg forstår det slet ikke.

Det usually means it or that, depending on context.

In real conversation, det can refer to:

• something just mentioned
• a situation
• an explanation
• a rule
• a joke
• what someone just said

For example:

• Someone explains grammar to you.
• You say: Jeg forstår det slet ikke.

Here det means it = the thing being explained.

Danish uses det very often, sometimes even where English might prefer something a bit more specific.

Yes, very natural.

Jeg forstår det slet ikke is a normal, idiomatic sentence that Danes really say. It works in:

• conversation
• class
• text messages
• everyday complaints
• reacting to something confusing

It is neither especially formal nor especially slangy. It is just standard, common Danish.

A rough guide for an English speaker:

• Jeg sounds somewhat like yai or yigh, depending on speaker and accent
• forstår roughly like fo-STOR
• det often sounds more like deh or a very soft d
• slet roughly like sled with a short vowel
• ikke roughly like IG-uh or ik-uh, though the real Danish sound is softer and harder to imitate exactly

A very rough full approximation:

• yai fo-STOR deh sled IG-uh

A few important pronunciation notes:

• Danish g in jeg is not a hard English g
• r in forstår is Danish, not strongly rolled
• det is often reduced in normal speech
• ikke is often pronounced more like ikk(e) than fully spelled-out

If you want to sound natural, listening to native audio is especially important with Danish.

Yes.

The neutral sentence is:

• Jeg forstår det slet ikke

But Danish can move elements to the front for emphasis:

• Det forstår jeg slet ikke

This means something like:

• That I really don’t understand
• or simply I really don’t understand that

When you move det to the front, Danish still keeps the verb in second position, so you get:

• Det
  • forstår
    • jeg
      • slet ikke

That is a key Danish word-order rule in main clauses.

Yes, in this kind of use it is tied to negation.

You normally use slet together with ikke to mean not at all.

Examples:

• Det er slet ikke svært = It’s not difficult at all
• Jeg kender ham slet ikke = I don’t know him at all
• Hun kommer slet ikke = She’s not coming at all

So when learners meet slet, they should usually think of the fixed expression slet ikke.

Yes. That is also possible.

overhovedet ikke also means not at all, and it is often very similar to slet ikke.

Compare:

• Jeg forstår det slet ikke
• Jeg forstår det overhovedet ikke

Both are natural. In many situations, slet ikke is the shorter, very common everyday choice.

Yes, mainly these points:

1. Verb form

   • Danish present tense often ends in -r: forstår

2. Pronoun placement

   • Danish says Jeg forstår det ikke, not the English-style word-for-word pattern you might expect

3. Fixed expression

   • slet ikke should be learned as a chunk meaning not at all

4. Pronunciation

   • Danish pronunciation is often much less phonetic than the spelling suggests

So although the sentence is short, it contains several very typical Danish features.