Ég vona að við sleppum við mikla umferð á morgun.

Because the two við are different words with different jobs:

• the first við is the pronoun we
• the second við is a preposition, part of the expression sleppa við

So að við sleppum við... means that we avoid / escape..., not that we we...

Here að means that and introduces a subordinate clause:

• Ég vona að... = I hope that...

It is not the infinitive marker to in this sentence.

English often drops that, but Icelandic normally keeps að here.

Sleppa við is a common expression meaning:

• to avoid
• to escape
• to get away without experiencing something unpleasant

So sleppum við mikla umferð means we avoid a lot of traffic / heavy traffic.

This is important because sleppa by itself can mean other things too, such as:

• let go
• release
• skip

So the preposition við changes the meaning a lot.

Because Icelandic often uses the present tense for future meaning when the time is clear from the context.

Here the future time is shown by á morgun = tomorrow, so sleppum naturally means we avoid / will avoid.

English does something similar:

• I hope we avoid traffic tomorrow

That is present in form, but future in meaning.

This is a very natural question, because after verbs of hoping, learners often expect the subjunctive.

The key point is that sleppum is one of those forms where the 1st person plural present looks the same in both indicative and subjunctive. So from this form alone, you cannot see a difference.

In other words, the sentence does not visibly tell you the mood just by the form sleppum.

With another subject, the contrast may be clearer. For example, a form like sleppi can look more obviously subjunctive than indicative sleppir.

Because við takes the accusative case, and the adjective has to agree with the noun.

• umferð is a feminine singular noun
• accusative singular of mikill is mikla
• nominative singular would be mikil

So:

• mikil umferð = nominative
• mikla umferð = accusative

That is why the sentence has við mikla umferð.

Because some Icelandic nouns have the same form in more than one case.

Umferð looks the same in nominative and accusative singular, so the case is shown more clearly by the adjective:

• mikil umferð
• mikla umferð

This is very common in Icelandic. Often the adjective or article makes the case easier to see than the noun itself.

Also, umferð is usually singular, like English traffic.

Á morgun means tomorrow.

It is a fixed everyday expression, and it is very important not to confuse it with í morgun, which means:

• this morning
• earlier this morning

So:

• á morgun = tomorrow
• í morgun = this morning

A little, yes. Icelandic allows some flexibility, especially with time expressions like á morgun.

But the given order is very natural and neutral:

• Ég vona að við sleppum við mikla umferð á morgun.

For a learner, this is the safest and most standard way to say it. Moving á morgun elsewhere is possible in some contexts, but the version you have is the straightforward one to learn first.