Þau urðu ástfangin í sumar.

Þau means they, but specifically the neuter plural form.

In Icelandic, þau is commonly used for:

• a mixed-gender group
• a male + female pair
• children
• a group where gender is unspecified or not important

So if this sentence is about, for example, one man and one woman, þau is exactly what you would expect.

By contrast:

• þeir = they for an all-male group
• þær = they for an all-female group

Urðu is the past tense plural of verða, which means to become.

So þau urðu ástfangin literally means they became in love, which is how Icelandic expresses what English usually says as they fell in love.

This is an important contrast:

• þau urðu ástfangin = they fell in love / they became in love
• þau voru ástfangin = they were in love

So urðu shows a change of state, not just a state.

Yes. It comes from the irregular verb verða.

Some useful past forms are:

• ég varð = I became
• þú varðst = you became
• hann/hún/það varð = he/she/it became
• við urðum = we became
• þið urðuð = you became
• þau urðu = they became

So urðu is just the expected plural past form, even though it looks quite different from the dictionary form verða.

Yes. Ástfangin is an adjective meaning in love.

The dictionary form is usually given as ástfanginn.

In this sentence it is a predicate adjective, meaning it comes after the verb and describes the subject:

• þau = the subject
• urðu = became
• ástfangin = in love

So the structure is basically: They became in love.

Because it has to agree with þau.

Since þau is neuter plural, the adjective also appears in the neuter plural form:

• masculine plural: ástfangnir
• feminine plural: ástfangnar
• neuter plural: ástfangin

So þau urðu ástfangin is correct agreement.

A detail that can confuse learners: ástfangin can also be a different grammatical form in other contexts, but here the meaning is clear from the subject þau.

Not strictly from the grammar alone.

Þau urðu ástfangin means they fell in love. In many real contexts, people will naturally understand this as they fell in love with each other, especially if the sentence is about a couple. But the sentence itself does not force that interpretation.

So the exact nuance depends on context.

Here í sumar means this summer or over the summer, depending on context.

With a past-tense verb like urðu, it refers to the summer that has already passed in the relevant time frame:

• Þau urðu ástfangin í sumar. = They fell in love this summer.

It is a very common Icelandic time expression.

Because Icelandic often uses season words by themselves in expressions like this.

So:

• í sumar = this summer
• í vetur = this winter
• í haust = this autumn

English often needs this, but Icelandic frequently does not.

So even though sumar by itself means summer, the whole expression í sumar often means this summer, not just in summer in a general sense.

In this expression, í sumar is treated as an accusative time expression.

That can be hard to notice, because sumar is a neuter noun and the nominative and accusative singular forms look the same.

So a learner may not be able to see the case directly here, but grammatically this is a standard temporal expression with í.

Yes.

The sentence Þau urðu ástfangin í sumar is perfectly normal, but you can also say: Í sumar urðu þau ástfangin.

That happens because Icelandic is a verb-second language. If you move í sumar to the front for emphasis or style, the finite verb urðu still stays in the second position.

So both are natural:

• Þau urðu ástfangin í sumar.
• Í sumar urðu þau ástfangin.