Við förum til ömmu á sunnudag.

Because Icelandic verbs change form to match the subject.

The base form is fara, meaning to go.
But with við meaning we, the present-tense form is förum.

So:

• að fara = to go
• við förum = we go / we are going

This is one of the first big differences from English: Icelandic verbs are more heavily conjugated.

Because til requires the genitive case in Icelandic.

The dictionary form is amma meaning grandmother / grandma, but after til it changes to ömmu.

So:

• amma = nominative form
• til ömmu = to grandma / to grandma’s place

This is very common in Icelandic: prepositions often control a specific case, and til always takes the genitive.

Here til means to, but with a nuance that often feels like to someone’s place or to visit someone.

So fara til ömmu is very natural Icelandic for:

• go to grandma
• or more idiomatically, go to grandma’s

English often leaves house or place unspoken too, and Icelandic does the same here.

You know from the preposition, not just from the ending.

With this noun, ömmu can be the form for several non-nominative cases. So the form by itself does not tell you everything. But til always takes the genitive, so in til ömmu, the case must be genitive.

In other words:

• the noun form alone can be ambiguous
• the preposition tells you which case is meant

This is very normal in Icelandic.

Because Icelandic commonly uses á with the day of the week to mean on a certain day.

Here the weekday is in the form sunnudag, so:

• á sunnudag = on Sunday

This is a very useful pattern:

• á mánudag = on Monday
• á þriðjudag = on Tuesday
• á föstudag = on Friday

So if you want to say on + weekday, this is a pattern worth memorizing.

It can mean any of those, depending on context.

In this sentence, because you already have the time expression á sunnudag, the present tense is naturally understood as a future plan:

• Við förum til ömmu á sunnudag = We are going to grandma’s on Sunday
• or We will go to grandma’s on Sunday

Icelandic often uses the present tense for planned or scheduled future events, just like English does in sentences such as We’re leaving on Sunday.

Because Icelandic does not need an article there.

In English, family words like grandma, mom, or dad are often used without the, and Icelandic works similarly in many contexts.

So til ömmu is perfectly natural and does not need anything equivalent to the grandma.

Also, in Icelandic, definiteness is often shown differently from English, sometimes with a suffix rather than a separate word. But in this sentence, no definite article is needed at all.

Yes.

A very natural alternative is:

Á sunnudag förum við til ömmu.

That still means the same thing: On Sunday, we’re going to grandma’s.

The important thing is that Icelandic usually keeps the finite verb in the second position of the clause. So if you move á sunnudag to the front, förum stays in second position, and við comes after it.

Compare:

• Við förum til ömmu á sunnudag.
• Á sunnudag förum við til ömmu.

Both are correct.

A rough learner-friendly guide would be:

• Við: the ð is like the th in this, though it can sound quite light
• förum: the ö is a rounded vowel, a bit like German ö or French eu; there is no exact normal English equivalent
• ömmu: same ö sound again
• sunnudag: stress is on the first syllable, as is usual in Icelandic

A very rough approximation might be:

vith FÖ-rum til UM-mu au SUN-nu-dag

But that is only approximate. The hardest sounds here for many English speakers are:

• ð
• ö

So those are worth practicing separately.