Se o ecrã estiver escuro, tens de carregar no botão para ligar o computador.

Because after se meaning if, Portuguese often uses the future subjunctive when talking about a possible future situation.

• Se o ecrã estiver escuro = If the screen is dark / if the screen turns out to be dark
• estiver is the future subjunctive of estar

This is very natural in Portuguese, even though English usually just uses the present tense:

• If the screen is dark, ...

So although English has is, Portuguese uses estiver here.

Estiver is a form of the verb estar (to be) in the future subjunctive.

Here is the full future subjunctive of estar:

• eu estiver
• tu estiveres
• ele/ela/você estiver
• nós estivermos
• vós estiverdes
• eles/elas/vocês estiverem

In this sentence, the subject is o ecrã, which is third person singular, so we use estiver.

Portuguese usually uses estar for temporary states or conditions, and ser for more permanent characteristics.

Here, the screen is dark means its current condition is dark, not that darkness is an essential characteristic of the screen. So:

• o ecrã está escuro = the screen is dark right now
• o ecrã é escuro would sound more like the screen is dark-coloured by nature

So estar is the correct choice.

Yes. Ecrã is the usual European Portuguese word for screen.

In Portugal:

• ecrã = screen

In Brazilian Portuguese, you are more likely to hear:

• tela = screen

So o ecrã is a very European Portuguese choice.

Yes. Tens de means you have to or you need to.

It comes from:

• ter de + infinitive = to have to do something

So:

• tens de carregar = you have to press

Because the sentence is addressing tu, the verb ter becomes tens.

Examples:

• Tens de estudar. = You have to study.
• Tenho de sair. = I have to leave.

In European Portuguese, ter de is very common and standard.

Because Portuguese often drops subject pronouns when the verb ending already shows who the subject is.

• tens clearly means you with tu
• so tu tens de carregar and tens de carregar both mean you have to press

Portuguese does this much more than English. The pronoun is only added when needed for emphasis, contrast, or clarity.

No is a contraction of:

• em + o = no

So literally:

• carregar no botão = press on the button

In European Portuguese, carregar em/no botão is the normal way to say press the button.

So:

• carregar no botão = press the button
• carregar na tecla = press the key

This is a very common European Portuguese pattern.

Not always. Carregar has several meanings, depending on context.

In this sentence:

• carregar no botão = to press the button

But carregar can also mean things like:

• to load
• to carry
• to charge (for example a battery, in some contexts)

So the meaning depends heavily on the words around it. Here, because of no botão, it clearly means press.

Yes. In Portuguese, ligar often means to switch on / turn on a device.

So:

• ligar o computador = turn on the computer

This verb is very common for devices:

• ligar a televisão = turn on the TV
• ligar o telemóvel = turn on the mobile phone

Be careful, though: ligar can also mean to call someone, depending on context. For example:

• Vou ligar-te. = I’m going to call you.

Here, with o computador, it means turn on.

Para here means to / in order to and introduces purpose.

So:

• carregar no botão para ligar o computador means
• press the button to turn on the computer or
• press the button in order to turn on the computer

It explains why you press the button.

Because it agrees with ecrã, which is a masculine singular noun:

• o ecrã → masculine singular
• therefore escuro → masculine singular

If the noun were feminine, the adjective would change:

• a sala está escura = the room is dark

So the adjective must match the noun in gender and number.

In this sentence, Se o ecrã estiver escuro is the correct and natural choice because it refers to a possible future situation.

• Se o ecrã estiver escuro, ... = If the screen is dark, ...

Using está after se is generally not the normal choice here in standard Portuguese when you mean a future possibility.

A learner should strongly prefer:

• se + future subjunctive for this kind of if clause about the future

So this is a key grammar pattern worth learning.

They are close in meaning, but ter de is the more standard and widely preferred form, especially in careful European Portuguese.

• tens de carregar = you have to press
• tens que carregar may be heard, but ter de is generally the safer choice for learners

So if you are learning Portuguese from Portugal, tens de is the better form to use.

It breaks down like this:

• Se = if
• o ecrã = the screen
• estiver escuro = is dark / turns out to be dark
• tens de = you have to
• carregar no botão = press the button
• para ligar o computador = to turn on the computer

So the structure is:

• If + condition, main action

More literally:

• If the screen is dark, you have to press the button to turn on the computer.

This is a very common Portuguese pattern:

• Se + future subjunctive, then the main clause