Við ætlum að sigla með ferjunni yfir sjóinn á morgun.

Ætlum is the we form of the verb ætla.

The pattern ætla að + infinitive means things like:

• intend to
• plan to
• be going to

So Við ætlum að sigla... means We intend to sail... or more naturally We’re going to sail...

Because Icelandic often uses the present tense to talk about the future, especially when the time is already clear from the sentence.

Here, the future meaning comes from:

• ætlum að = planned/intended action
• á morgun = tomorrow

So even though ætlum is present tense grammatically, the sentence clearly refers to a future plan.

Here að is the infinitive marker, like English to in to sail.

So:

• sigla = sail
• að sigla = to sail

After ætla, Icelandic normally uses að + infinitive:

• ætla að fara = intend to go
• ætla að borða = intend to eat
• ætla að sigla = intend to sail

Its basic meaning is to sail or travel by boat/ship.

In this sentence, sigla með ferjunni can naturally suggest traveling on the ferry, but it still sounds more nautical than a very general go.

A useful nuance:

• sigla focuses on the sea travel itself
• fara með ferjunni is often the more everyday way to say go by ferry / take the ferry

So this sentence is perfectly understandable, but it has a slightly more sailing/traveling by sea feel than the plainest everyday phrasing.

Með usually means with, but in many contexts it also describes the means of transport, similar to English by or on.

So með ferjunni means something like:

• by ferry
• on the ferry
• with the ferry in a very literal sense

In English, with the ferry sounds odd, but in Icelandic this use of með is normal.

Because með takes the dative case, and ferjunni is the dative singular definite form of ferja.

A simple breakdown:

• ferja = ferry
• ferju = ferry, dative singular
• ferjunni = the ferry, dative singular

So með ferjunni literally means with/by the ferry.

This is also a good example of how Icelandic puts the definite article onto the end of the noun instead of using a separate word like English the.

Because yfir can take different cases depending on the meaning.

A very common rule is:

• accusative for motion/direction
• dative for location/static position

Here the idea is movement across the sea, so Icelandic uses the accusative:

• yfir sjóinn = across the sea

If it were describing something being located over the sea rather than moving across it, you would expect a dative form instead.

Sjór is an irregular masculine noun, and here it appears in the accusative singular definite form.

So:

• sjór = sea, nominative singular
• sjó = sea, accusative singular
• sjóinn = the sea, accusative singular definite

This form is used because:

1. yfir here requires the accusative
2. the noun is definite: the sea

Icelandic often uses the definite form when the speaker has a specific, known thing in mind.

So here, the sentence can suggest:

• a particular ferry
• a particular stretch of sea being crossed

That does not always match English exactly. English sometimes uses the, but sometimes prefers a more general phrasing.

For example:

• með ferju = by ferry, in general
• með ferjunni = by/on the ferry, a specific ferry

So the definite forms make the sentence feel more specific.

Because they give different kinds of information.

• með ferjunni tells you how the trip happens
• yfir sjóinn tells you where/across what the movement goes

So the sentence is not repeating itself.

Compare the difference:

• Við ætlum að sigla yfir sjóinn á morgun.
  We’re going to sail across the sea tomorrow.

• Við ætlum að fara með ferjunni á morgun.
  We’re going to go by ferry tomorrow.

• Við ætlum að sigla með ferjunni yfir sjóinn á morgun.
  We’re going to travel by/on the ferry across the sea tomorrow.

Yes. Á morgun is the standard expression for tomorrow.

You should mostly learn it as a fixed phrase rather than translating it word for word.

A useful contrast is:

• á morgun = tomorrow
• í morgun = this morning

So even though both contain morgun, the whole expressions mean different things.

Yes. Icelandic word order is flexible, but it follows an important verb-second pattern in main clauses.

Your sentence has the normal order:

• Við ætlum að sigla með ferjunni yfir sjóinn á morgun.

But you can move the time phrase to the front:

• Á morgun ætlum við að sigla með ferjunni yfir sjóinn.

Notice what happens there:

• the time expression comes first
• the finite verb ætlum still stays in the second position
• við moves after the verb

That is a very common Icelandic pattern.