Ekki klúðra pöntuninni.

Because this is a negative command: Ekki means not / don’t here.

In Icelandic, a very common way to make a negative command is:

Ekki + infinitive

So:

Ekki klúðra ... = Don’t mess up ...

This is a very natural pattern in everyday Icelandic, similar to Ekki fara! = Don’t go!

Formally, klúðra is the infinitive of the verb, from að klúðra.

In negative commands, Icelandic often uses the infinitive after Ekki, so even though the sentence is giving a command, the verb appears in its infinitive form.

Compare:

• Ekki klúðra pöntuninni. = common negative command
• Klúðraðu ekki pöntuninni. = also correct, more explicitly addressed to one person

So the sentence is imperative in meaning, but klúðra itself is infinitive in form.

It is simply understood.

Like English imperatives, Icelandic commands usually leave out the subject:

• Eat!
• Don’t go!

You do not normally say you unless you want emphasis.

So in Ekki klúðra pöntuninni, the understood subject is you.

Klúðra is a very common everyday verb meaning things like:

• mess up
• botch
• screw up

It is somewhat colloquial, but extremely normal in everyday speech. It sounds much more natural in casual conversation than a very formal verb would.

So if someone says Ekki klúðra pöntuninni, the tone is everyday and direct, not especially formal.

Because klúðra takes the dative case, not the accusative.

This is one of those Icelandic verbs where you have to learn the case that goes with it:

að klúðra einhverju = to mess something up

So:

• pöntuninni = dative singular definite
• pöntunina = accusative singular definite

With klúðra, standard Icelandic uses the dative, so pöntuninni is the correct form here.

The base noun is pöntun, a feminine noun meaning order in the sense of an order for food, goods, services, etc.

Here is the relevant idea:

• pöntun = an order
• pöntunin = the order in nominative
• pöntuninni = the order in dative

So pöntuninni is the same noun, just in a different case and with the definite article attached.

It is part of the definite article plus the case ending.

In Icelandic, the is usually not a separate word. Instead, it is attached to the end of the noun.

So pöntuninni breaks down roughly like this:

• pöntun- = the noun stem
• -inni = the definite dative singular ending for this feminine noun

That is why Icelandic does not need a separate word like English the here.

Not every kind.

Pöntun usually means an order in the sense of ordering food, products, services, or sometimes a booking/request depending on context.

It does not usually mean order in the sense of a command. For that, Icelandic would more likely use words such as skipun or fyrirmæli.

So in this sentence, pöntun is the kind of order you place, not the kind you give.

Yes. That is also natural Icelandic.

The difference is mainly in form:

• Ekki klúðra pöntuninni. = common negative command using Ekki + infinitive
• Klúðraðu ekki pöntuninni. = negative command with an explicit singular imperative

The first version is very common and straightforward. The second makes the command more clearly directed at one person.

A few possibilities are natural.

The sentence Ekki klúðra pöntuninni can already work as a general command without clearly marking singular or plural.

If you want to make the plural explicit, you can use the plural imperative:

• Klúðrið ekki pöntuninni. = Don’t mess up the order. said to more than one person

For one person, you would use:

• Klúðraðu ekki pöntuninni.

So the sentence you were given is neutral and common, while the imperative forms can show number more clearly.