O ciúme deixa o casal nervoso.

In Portuguese, it is very common to use the definite article with abstract nouns, especially when talking about something in a general sense.

So o ciúme means jealousy as a concept here, not necessarily the jealousy in a very specific English sense.

This is one of those places where Portuguese often sounds more natural with an article, while English often leaves it out.

• O ciúme faz mal. = Jealousy is harmful.
• O amor é importante. = Love is important.

Here, deixa comes from deixar, and it means something like makes or leaves in the pattern:

deixar + object + adjective

So:

• O ciúme deixa o casal nervoso.
• literally: Jealousy leaves the couple nervous
• natural English: Jealousy makes the couple nervous

This is a very common structure in Portuguese:

• Isso me deixa triste. = That makes me sad.
• O barulho deixa ela irritada. = The noise makes her irritated/annoyed.

Because the subject is o ciúme, which is singular.

Even though o casal refers to two people, it is not the subject here. It is the object.

Sentence structure:

• O ciúme = subject
• deixa = verb
• o casal = object
• nervoso = adjective describing o casal

So the verb agrees with o ciúme, not with o casal.

Because casal is a singular noun in Portuguese, just like couple in English.

It refers to two people together as one unit.

So:

• o casal = the couple
• singular grammar, even though it contains two people

That is why the sentence uses:

• o casal
• nervoso (singular)

not plural forms.

Because nervoso agrees with o casal, and casal is singular masculine.

In Portuguese, adjectives normally agree in gender and number with the noun they describe.

So:

• o casal nervoso = the nervous couple
• singular masculine

Even though the couple contains two people, the grammar follows the singular noun casal.

If you talked about the two people directly, then you would use the plural:

• Os dois estão nervosos. = The two of them are nervous.

Not always.

In Brazilian Portuguese, nervoso can mean:

• nervous
• tense
• upset
• irritated
• agitated

So in this sentence, nervoso could mean more than just shy or anxious. It may suggest emotional tension caused by jealousy.

That broader meaning is very common in Portuguese.

No, not in this sentence.

The adjective must agree with casal, which is a masculine singular noun:

• o casal nervoso

Even if the couple is made up of a man and a woman, the noun itself is masculine, so the adjective stays masculine singular.

Both forms exist.

• ciúme can be used as an abstract or general idea: jealousy
• ciúmes is also very common in everyday Portuguese when talking about jealous feelings

For example:

• O ciúme atrapalha relacionamentos. = Jealousy gets in the way of relationships.
• Ela sente ciúmes dele. = She is jealous of him / She feels jealous of him

In your sentence, the singular ciúme sounds natural because it refers to jealousy as a general force or feeling.

Because deixar and ficar do different jobs here.

• deixar = to make someone become a certain way
• ficar = to become / to end up being

So:

• O ciúme deixa o casal nervoso. = Jealousy makes the couple nervous.
• O casal fica nervoso. = The couple gets nervous.

The first sentence emphasizes the cause: jealousy.

The second emphasizes the result: the couple becomes nervous.

Yes, you could, but it is not exactly the same structure or emphasis.

• O ciúme deixa o casal nervoso.
  Focus: jealousy causes the nervousness.

• O casal fica nervoso com ciúme.
  Focus: the couple gets nervous because of jealousy.

Both are natural, but the original sentence is a little more direct about jealousy being the cause.

Yes.

The accent mark in ciúme shows that the stress falls on ú:

• ci-Ú-me

That is especially helpful because the spelling might otherwise be confusing to an English speaker.

So the word is pronounced roughly like see-OO-meh, though the exact Portuguese sounds are different from English.

Yes.

In Brazilian Portuguese, casal usually means a couple in the romantic sense, and it does not have to mean they are married.

So it can refer to:

• husband and wife
• boyfriend and girlfriend
• partners
• a romantic pair in general

The exact meaning depends on context.