São sete horas.

Portuguese uses the verb ser to tell time.

In São sete horas, the understood idea is seven hours, and horas is plural, so the verb is plural too:

• É uma hora. = It’s one o’clock.
• São duas horas. = It’s two o’clock.
• São sete horas. = It’s seven o’clock.

So the basic rule is:

• 1:00 → é
• 2:00 and after → são

Also:

• É meio-dia.
• É meia-noite.

These are singular expressions.

Because Portuguese literally expresses clock time as hours.

So:

• uma hora = one hour / one o’clock
• duas horas = two o’clock
• sete horas = seven o’clock

That is why horas is plural with numbers above one.

Yes. In everyday Brazilian Portuguese, São sete is very common and natural when the context is clearly about time.

Examples:

• Que horas são?
• São sete.

You can still say São sete horas, and it sounds correct and normal too. The version with horas is just a little fuller.

Portuguese simply does not need a dummy subject like English it in this expression.

English says:

• It is seven o’clock.

Portuguese says:

• São sete horas.

The verb appears without a word equivalent to English it. This is very common in Portuguese.

Because when you are stating the time, you say:

• São sete horas.

But when you use a preposition such as at, Portuguese uses às:

• A aula começa às sete horas.
• Chego às sete.

So:

• São sete horas. = stating the time
• às sete horas = at seven o’clock

This difference is very important.

Brazilian Portuguese often uses parts of the day instead of AM/PM:

• São sete da manhã. = 7 AM
• São sete da noite. = 7 PM

You may also hear:

• da tarde for afternoon times
• da noite for evening/night times

Examples:

• São duas da tarde.
• São nove da noite.

In formal contexts, people may also use the 24-hour clock:

• São dezenove horas. = 19:00 / 7 PM

A broad Brazilian pronunciation is approximately:

• São → sounds roughly like sowng, with a nasal ending
• sete → roughly SEH-chee in many parts of Brazil, though some regions say more like SEH-tee
• horas → roughly OH-ras, with a light Portuguese r

A few useful notes:

• ão in São is nasal, which is often difficult for English speakers.
• In many Brazilian accents, te can sound like chee, so sete may sound like SEH-chee.
• The h in horas is silent.

The most common question is:

• Que horas são? = What time is it?

So a natural exchange is:

• Que horas são?
• São sete horas.

You may also hear the shorter answer:

• São sete.

Then Portuguese switches to singular:

• É uma hora.

Not:

• São uma hora.

This is because uma hora is singular, so the verb must also be singular.

Compare:

• É uma hora.
• São duas horas.
• São sete horas.

Yes, especially in conversation. If someone asks the time, a short answer like Sete horas or even just Sete can sound perfectly natural.

For example:

• Que horas são?
• Sete horas.

Still, São sete horas is the full standard sentence and is always safe to use.

It works in both formal and casual Brazilian Portuguese.

• São sete horas. = complete, neutral, correct in any context
• São sete. = slightly shorter, very common in speech
• Sete. = very casual, only when the context is obvious

So if you are unsure, São sete horas is an excellent default.