Prošlog ljeta nas je iznenadilo nevrijeme s jakim vjetrom i tučom.

Prošlog ljeta is genitive singular, and Croatian very often uses the genitive in expressions of time like:

• prošlog tjedna = last week
• ove godine = this year
• prošlog ljeta = last summer

So here it functions as a time expression meaning last summer or during last summer.

Nas means us.

Here it is the direct object, so it is functioning as the accusative form of mi = we.

In other words:

• nevrijeme = the thing doing the action
• nas = the people affected by it

So literally: Bad weather surprised us.

Croatian usually forms the past tense with:

• a present-tense form of biti = to be
• plus the l-participle

So:

• je iznenadilo = surprised

This is the normal Croatian perfect tense pattern, not something extra or redundant.

Examples:

• je došao = he came
• je pala = it fell / she fell
• je iznenadilo = it surprised

Because the verb form agrees with the subject, and the subject here is nevrijeme.

Nevrijeme is a neuter singular noun, so the participle is also neuter singular:

• nevrijeme je iznenadilo
• not iznenadio
• not iznenadila

Compare:

• vjetar je iznenadio = the wind surprised
• oluja je iznenadila = the storm surprised
• nevrijeme je iznenadilo = the bad weather surprised

Croatian word order is much more flexible than English word order.

In this sentence, nas je are unstressed words (a pronoun + auxiliary), and Croatian usually places such clitics near the beginning of the sentence, often after the first phrase:

• Prošlog ljeta | nas je | iznenadilo nevrijeme

So the order is natural Croatian word order, not a mistake.

A version like Prošlog ljeta nevrijeme nas je iznenadilo is also possible, but the given sentence sounds very natural.

Nevrijeme usually means bad weather, severe weather, or a stormy weather event.

It is stronger than just vrijeme:

• vrijeme = weather / time
• nevrijeme = bad weather, rough weather, stormy conditions

In this sentence, because it is followed by s jakim vjetrom i tučom, it clearly means something like severe weather / a storm with strong wind and hail.

Because the preposition s = with takes the instrumental case.

So:

• s jakim vjetrom = with strong wind
• s tučom = with hail

The forms are:

• jak → jakim
• vjetar → vjetrom
• tuča → tučom

So this whole part means with strong wind and hail.

Grammatically, jakim directly modifies only vjetrom.

So the phrase most literally means:

• with strong wind and hail

If you wanted to say that both were strong/intense, Croatian would normally repeat the adjective:

• s jakim vjetrom i jakom tučom

That said, in the original sentence the meaning is still very natural: the bad weather included strong wind and hail.

Both s and sa mean with.

Sa is usually used when it is easier to pronounce, especially before certain consonants or consonant clusters. Before jakim, simple s is the normal choice:

• s jakim vjetrom = natural
• sa jakim vjetrom = not the usual form here

So this is mainly a pronunciation/phonetic issue, not a difference in meaning.

Yes. In weather context, tuča means hail.

So:

• s jakim vjetrom i tučom = with strong wind and hail

This is a common weather word in Croatian, especially in reports about storms or severe weather.