Il mio amico era seccato di non poter nuotare in mare a causa del maltempo.

In standard Italian, possessive adjectives usually come with the definite article:

• il mio libro
• la mia casa
• i miei amici

The main common exception is with many singular close family members, such as:

• mio padre
• mia madre
• mio fratello

Amico is not normally part of that exception, so il mio amico is the standard form here.

A useful contrast:

• il mio amico = my friend
• un mio amico = a friend of mine

Era is the imperfetto of essere. Italian often uses the imperfect to describe a past state, feeling, or background situation.

Here, era seccato describes how the friend felt at that time: it is a past emotional state.

• è seccato = he is annoyed / present
• era seccato = he was annoyed / past state
• è stato seccato can sound more like a completed event, and in some contexts it may suggest someone annoyed him

For emotions and states in a past situation, era is very natural.

Here seccato means something like:

• annoyed
• bothered
• irritated
• fed up
• sometimes more mildly, bummed or put out

In this sentence, it probably sounds more like annoyed/disappointed than deeply angry.

Italian seccare is a very common verb in spoken language for to annoy or to bother:

• Mi secca = It annoys me / I can’t be bothered

So era seccato is a very natural way to say that he was feeling annoyed.

After many adjectives or participles expressing feelings, Italian often uses di + infinitive to explain what someone feels something about, especially when the subject is the same.

So:

• era seccato di non poter nuotare

means he was annoyed about not being able to swim.

This pattern is common:

• felice di vederti = happy to see you
• contento di partire = glad to leave
• stanco di aspettare = tired of waiting

Here, the same person is both:

• the one who is annoyed
• the one who cannot swim

That is why the infinitive structure works well.

There are two useful things here.

1. Why is non before poter? Because the negation applies to the idea of being able.

• non poter nuotare = to not be able to swim
• poter non nuotare = to be able not to swim / to be allowed to skip swimming

Those mean different things, so the position of non matters.

2. Why poter and not potere? Italian infinitives sometimes drop the final -e before another infinitive, especially with modal verbs:

• poter nuotare
• dover partire
• saper parlare

So poter nuotare is a very common shortened form of potere nuotare. Both are possible, but the shorter form is often more natural.

This is a very common and important distinction.

• in mare = in the sea, actually in the water
• al mare = at the seaside, by the sea, or to the beach/seaside area
• nel mare = more specific or literal, often in the sea as a particular body of water

So:

• nuotare in mare = to swim in the sea
• andare al mare = to go to the seaside

In this sentence, the friend could not swim in the sea water, so in mare is the natural choice.

A causa di is a fixed expression meaning:

• because of
• due to

So:

• a causa del maltempo = because of the bad weather / due to the bad weather

Why del? Because it comes from:

• di + il = del

Why maltempo as one word? Because maltempo is a standard Italian noun meaning bad weather, rough weather, or stormy conditions. It is very common and more idiomatic than trying to build the phrase word by word.

Yes. Per il maltempo is also very natural.

The difference is mostly one of style:

• a causa del maltempo = a bit more explicit or slightly more formal
• per il maltempo = shorter, simpler, very common in everyday speech

So both are fine in this context.

Yes, absolutely. That is a perfectly natural sentence.

Compare the two:

• era seccato di non poter nuotare = more compact, uses an infinitive
• era seccato perché non poteva nuotare = more explicit, uses a full clause

Italian often prefers the infinitive when the subject is the same, but the perché version is very common and may feel a little more straightforward to learners.

Because seccato agrees with amico, which is masculine singular.

Agreement works like this:

• il mio amico era seccato = masculine singular
• la mia amica era seccata = feminine singular
• i miei amici erano seccati = masculine plural or mixed plural
• le mie amiche erano seccate = feminine plural

So the ending changes to match the noun it refers to.