Ekki spilla endinum!

Because Icelandic very often forms a negative command with ekki + infinitive.

So:

• Ekki spilla endinum! = a normal way to say Don’t spoil the ending!

This is different from English, where we usually use don’t + base verb. In Icelandic, this ekki + infinitive pattern is extremely common for warnings, instructions, and general prohibitions.

Yes. That is also possible.

There are two useful patterns to know:

• Ekki + infinitive → Ekki spilla endinum!
• Imperative + ekki → Spilltu ekki endinum!

The difference is mostly one of style and directness:

• Ekki spilla endinum! sounds like a general prohibition or warning.
• Spilltu ekki endinum! sounds more directly addressed to one person.

If speaking to several people, you could say:

• Spillið ekki endinum!

So the sentence you were given is not the only possible form, but it is a very natural one.

Because the verb spilla takes a dative object.

That is something learners simply have to memorize with the verb:

• spilla einhverju = to spoil/ruin something

So even though the noun is the object in meaning, it is not in the accusative. It is in the dative:

• endinum = the ending in dative singular definite

This is very common in Icelandic: some verbs govern accusative objects, some dative, and some genitive.

The dictionary form is endir.

Here is the rough path:

• endir = ending, end
• dative singular indefinite: endi
• dative singular definite: endinum

So endinum means the ending in the form required by spilla.

Here, -num is part of the definite article in the dative singular masculine.

Icelandic usually attaches the to the end of the noun instead of putting a separate word in front, so:

• endir = an ending / ending
• endinum = the ending, in dative singular

It is important not to think of -num as a universal ending meaning just the. It specifically reflects:

• definiteness
• case
• number
• gender

So the full form changes depending on grammar.

In the sense used here, yes: the thing being spoiled is normally in the dative.

For example:

• spilla matnum = spoil the food
• spilla leiknum = ruin the game
• spilla endinum = spoil the ending

So when you learn spilla, it is a good idea to learn it as:

• spilla + dative

That will save you a lot of case mistakes later.

One nice thing about the ekki + infinitive pattern is that it does not clearly mark singular or plural.

So Ekki spilla endinum! can work as:

• a general warning
• a command to one person
• a command to several people
• even something like a notice or instruction

If you want to be more explicit:

• singular: Spilltu ekki endinum!
• plural: Spillið ekki endinum!

But the original sentence is perfectly natural as a more neutral prohibition.

In this kind of negative command, ekki normally comes first because the structure is:

• ekki + infinitive

So:

• Ekki spilla...

If you want the verb first, you normally switch to an actual imperative form:

• Spilltu ekki...
• Spillið ekki...

So Ekki spilla... and Spilltu ekki... are both possible, but they belong to slightly different grammatical patterns.

Yes, it is grammatical and natural for the idea of spoiling an ending.

That said, in everyday conversation Icelanders also often use other expressions depending on the context, especially if they mean spoiling something for someone. For example, you may hear phrases built around skemma or skemma fyrir in more conversational speech.

Still, Ekki spilla endinum! is a good, clear sentence and teaches an important pattern:

• negative command
• verb that takes dative
• definite noun with attached article