Skrifaðu þetta niður.

Skrifaðu is the singular imperative of skrifa (to write).

It is the form you use when telling one person to do something:

• skrifa = to write
• Skrifaðu! = Write!

So in this sentence, the speaker is giving a direct instruction to one person.

The dictionary form is skrifa.

However, in this sentence the full idea is really skrifa niður, which works like write down / note down.

So if you look it up, you may find:

• skrifa = write
• skrifa niður = write down, note down

That -ðu is a very common ending in the singular imperative of Icelandic verbs.

For many verbs, when you tell one person to do something, you get forms like:

• komdu = come
• farðu = go
• skrifaðu = write

So the -ðu ending is a good thing to recognize as a command to one person.

Þetta is a demonstrative pronoun, meaning this.

In this sentence, it is the direct object of the verb: it is the thing being written down.

A useful detail for learners is that þetta is neuter singular, and here it is in the accusative role. In practice, the neuter singular form þetta looks the same in nominative and accusative, so you do not see a change in the word itself.

Niður literally means down, but here it functions as a particle/adverb that combines with skrifa.

So skrifa niður means not just write, but specifically write down or note down.

That is why niður is important: it adds the idea of recording something for later.

Yes, but the meaning changes slightly.

• Skrifaðu þetta. = Write this.
• Skrifaðu þetta niður. = Write this down / Note this down.

Without niður, the sentence is more general. With niður, it sounds more like making a note of something.

In this kind of sentence, Skrifaðu þetta niður is the most natural everyday order.

Here, niður behaves like a verbal particle, and Icelandic often places the object before that particle in ordinary speech:

• Skrifaðu þetta niður.

Other orders may be possible in some contexts, especially for emphasis, but this is the standard, natural version for learners to copy.

To more than one person, use the plural imperative:

• Skrifið þetta niður.

So:

• Skrifaðu þetta niður. = said to one person
• Skrifið þetta niður. = said to two or more people

It is a normal direct command to one person.

Icelandic usually uses þú much more freely than many European languages use their informal you, so this does not sound unusually rude by itself. Still, as in English, a bare imperative can sound quite direct depending on tone.

If you want to sound softer, Icelandic often uses a different structure, for example:

• Geturðu skrifað þetta niður? = Can you write this down?
• Viltu skrifa þetta niður? = Would you like to write this down? / Will you write this down?

These two letters are very important in Icelandic:

• þ is like the th in thing
• ð is like the th in this

So in this sentence:

• þetta begins with the thing sound
• Skrifaðu contains ð, the voiced th sound

A rough learner-friendly approximation would be:

• Skrifaðu þetta niður
• SKRI-va-thu THET-ta ni-thur

That is only approximate, but it helps you notice the two different th sounds.

Yes. It is a very natural and useful everyday sentence.

You can use the same pattern with other objects:

• Skrifaðu nafnið niður. = Write the name down.
• Skrifaðu númerið niður. = Write the number down.
• Skrifaðu þetta hjá þér. = Write this down for yourself / make a note of this

So this sentence is a good model for giving simple practical instructions.