Ekki öskra í stofunni; amma sefur.

Because this is a bare infinitive. In dictionaries, the verb is usually given as að öskra. But after ekki in a negative command, Icelandic normally drops að.

So:

• að öskra = to shout
• Ekki öskra = Don’t shout

It works a bit like English don’t shout, not don’t to shout.

Yes — it is very normal and very common.

In Icelandic, one frequent way to tell someone not to do something is:

• ekki + bare infinitive

Examples:

• Ekki fara! = Don’t go!
• Ekki snerta þetta! = Don’t touch this!
• Ekki öskra! = Don’t shout!

So this sentence uses a standard everyday pattern.

Yes, that is also possible.

• Ekki öskra = very common, neutral prohibition
• Öskraðu ekki = imperative form + negation

The singular imperative of öskra is öskraðu. Both versions can mean don’t shout, but Ekki öskra is often the simpler and more common pattern learners meet first.

Because í changes case depending on whether you mean location or movement.

• dative = being in a place
• accusative = moving into a place

Here the meaning is in the living room, so it is location, not movement. That is why Icelandic uses the dative:

• í stofunni = in the living room

Compare:

• inn í stofuna = into the living room

This is a very important Icelandic pattern with several prepositions.

It shows two things at once:

1. the noun is definite = the living room
2. the noun is in the dative singular

The base noun is stofa.

A few forms are:

• stofa = living room, sitting room
• stofan = the living room (nominative)
• stofuna = the living room (accusative)
• stofunni = the living room (dative)

So Icelandic usually adds the as an ending on the noun instead of using a separate word.

Usually stofa means living room, sitting room, or lounge.

Depending on context or translation style, you may also see older words like parlor, but for most learners living room is the best everyday equivalent.

So í stofunni most naturally means in the living room.

Sefur is the 3rd person singular present tense of að sofa = to sleep.

So:

• ég sef = I sleep / I am sleeping
• þú sefur = you sleep / you are sleeping
• amma sefur = grandma sleeps / grandma is sleeping

It matches amma because amma is singular.

Because Icelandic simple present often covers both:

• a general/habitual meaning
• an action happening right now

So amma sefur can naturally mean Grandma is sleeping if the context is about what is happening now.

Icelandic also has a progressive-style expression:

• amma er að sofa

That also means Grandma is sleeping, often with a stronger sense of an action in progress. But the simple present amma sefur is completely normal here.

Icelandic usually does not use a separate word like English the. Instead, definiteness is often shown by an ending on the noun.

But in this sentence, amma is not definite in the sense of the grandmother. It is being used more like a family title or name: Grandma.

So:

• amma = Grandma / grandma
• amman = the grandmother

In family context, amma without the definite ending is very natural.

Because Icelandic normally keeps kinship words like amma, afi, mamma, and pabbi in lowercase unless they begin the sentence.

English sometimes capitalizes Grandma when it is used almost like a name, but Icelandic usually does not do that in the same way.

So amma sefur is perfectly normal.

The semicolon links two closely related independent clauses:

• Ekki öskra í stofunni
• amma sefur

The second clause explains the reason for the first. It works much like English punctuation.

You could also make the relationship more explicit with a connector such as því að:

• Ekki öskra í stofunni því að amma sefur.

That means Don’t shout in the living room because Grandma is sleeping.

It is basically:

• Ekki + infinitive + place
• ;
• subject + verb

So:

• Ekki öskra = Don’t shout
• í stofunni = in the living room
• amma sefur = Grandma is sleeping

That makes the sentence easy to break into two parts:

1. a negative command
2. a short explanation for that command