Við verðum að fara inn, því það rignir mikið.

Verðum is the 1st person plural present tense form of verða.

In this sentence, verða að + infinitive means to have to / must.

So:

• við verðum að = we have to
• fara = go

Together: Við verðum að fara inn = We have to go inside.

Be careful: verða can also mean become in other contexts, but with að + infinitive it usually means have to.

After verða when it means have to, Icelandic uses að + infinitive.

So the pattern is:

• verða að gera eitthvað = have to do something

Examples:

• Ég verð að fara. = I have to go.
• Við verðum að bíða. = We have to wait.

Here:

• verðum að fara = have to go

This að is not the same as the English infinitive marker to in every situation, but in this structure it often corresponds neatly to to.

Inn means in / inside, but specifically with a sense of movement into somewhere.

That is why it is used with fara:

• fara inn = go in / go inside

By contrast, inni usually describes location, not movement:

• vera inni = be inside

So:

• Við verðum að fara inn. = We have to go inside.
• Við erum inni. = We are inside.

A useful contrast:

• inn = into, inward movement
• inni = inside, already there

In Icelandic, weather verbs often use það as a dummy subject, much like English it in it is raining.

So:

• það rignir = it rains / it is raining

The það does not refer to a specific thing. It is just grammatically required in this kind of sentence.

Other similar examples:

• Það snjóar. = It is snowing.
• Það er kalt. = It is cold.

So það here works very much like English it in weather expressions.

It can mean either, depending on context.

Rignir is the present tense of rigna = to rain. Icelandic present tense often covers both:

• it rains
• it is raining

In this sentence, because of the context, English would most naturally say:

• because it is raining heavily or
• because it’s raining a lot

So even though the Icelandic is just simple present, the best English translation may be progressive.

Here mikið is being used adverbially, meaning something like a lot, heavily, or very much.

So:

• það rignir mikið = it rains a lot / it is raining heavily

This can feel a little unusual to English speakers, because mikið is formally the neuter form of mikill = big / much / great, but Icelandic often uses forms like this adverbially.

In natural English, you might translate it as:

• heavily
• a lot

depending on what sounds best in context.

Yes, here því means because.

So:

• ..., því það rignir mikið. = ..., because it is raining heavily.

This is a very common way to introduce a reason in Icelandic.

Learners should note that því can also appear in other meanings or expressions in Icelandic, so its meaning depends on context. But in this sentence, it clearly means because.

Yes. Af því að is another very common way to say because.

So these are both possible:

• Við verðum að fara inn, því það rignir mikið.
• Við verðum að fara inn af því að það rignir mikið.

Both mean essentially the same thing.

Very roughly:

• því can sound a bit shorter or more compact
• af því að is also extremely common in everyday speech

As a learner, it is useful to recognize both.

In the clause after því, the normal order is used:

• það = subject
• rignir = verb
• mikið = adverb

So:

• það rignir mikið

Icelandic often has verb-second word order in main clauses, but subordinate clauses commonly have more regular subject + verb order. After því meaning because, það rignir mikið is the natural arrangement.

Yes. A very natural English translation is:

• We have to go inside
• We must go inside

Both work.

The Icelandic verða að often expresses necessity in a way that overlaps with both English have to and must. In everyday English, have to is often the most neutral choice, but must can also fit well depending on tone.

A rough English-friendly approximation is:

• Vith VER-thum ath FA-ra inn, thvee that RIG-nir MI-kith

A few notes:

• ð in við, verðum, and því is like the th in this
• þ in það is like the th in thin
• r is rolled or tapped
• í is a long ee sound
• a in fara is more like a in father

This is only approximate, but it can help you get started.

A good vocabulary breakdown is:

• við = we
• verðum = have to / must are required to; form of verða
• að = to, used here before the infinitive
• fara = go
• inn = in, inside, inward
• því = because
• það = it
• rignir = rains / is raining
• mikið = much, a lot, heavily

A very useful chunk to memorize is:

• verða að + infinitive = have to + verb

That pattern appears constantly in Icelandic.