Eu preciso verificar se a porta está fechada antes de sair.

Yes. In Brazilian Portuguese, eu can often be omitted because the verb form already shows the subject:

• Eu preciso verificar...
• Preciso verificar...

Both are natural. Including eu adds a little emphasis or clarity, but it is not required here.

After precisar when it means to need to, the next verb stays in the infinitive:

• preciso verificar = I need to check
• precisamos sair = we need to leave
• ela precisa estudar = she needs to study

So preciso verifico is incorrect because verifico is a conjugated form, and Portuguese does not normally stack two conjugated verbs like that in this structure.

Here preciso is the first-person singular of precisar:

• eu preciso = I need

So Eu preciso verificar... means I need to check...

Be careful: preciso can also be an adjective meaning precise or accurate in other contexts, but here it is clearly the verb.

Here se introduces an indirect yes/no question, so it means if or whether:

• verificar se a porta está fechada = check if / whether the door is closed

This is a very common structure in Portuguese:

• Não sei se ele vem. = I don’t know if/whether he is coming.
• Perguntei se estava aberto. = I asked if/whether it was open.

So this se is not reflexive and not conditional in the English sense of if X happens, Y happens. It is marking an embedded question.

Portuguese uses definite articles much more often than English does. So where English says the door or sometimes just door in certain contexts, Portuguese usually prefers a porta.

In this sentence, a porta is completely natural because it refers to a specific door relevant in the situation.

Compare:

• Feche a porta. = Close the door.
• A janela está aberta. = The window is open.

Leaving out the article here would sound unnatural.

Portuguese usually uses estar for a temporary state or condition, and ser for identity, essence, or more permanent characteristics.

Here, the door is closed is a current condition, so estar is the normal choice:

• A porta está fechada. = The door is closed.

If you said A porta é fechada, it would usually sound odd in this context, or it could suggest something more like the door is of the closed type or some unusual descriptive meaning, not the normal state of being shut right now.

So for open/closed in ordinary situations, use:

• está aberta
• está fechada

Because it agrees with porta, which is a feminine singular noun.

• a porta → feminine singular
• fechada → feminine singular form of fechado

Agreement is very important in Portuguese. Compare:

• A porta está fechada.
• O portão está fechado.
• As portas estão fechadas.
• Os portões estão fechados.

So the ending changes to match the noun.

You can think of it as the past participle of fechar being used adjectivally.

• fechar = to close
• fechado / fechada = closed

In A porta está fechada, it functions like an adjective describing the door’s state.

This is very common in Portuguese:

• A janela está aberta.
• O trabalho está terminado.
• A loja está fechada.

So grammatically it comes from a participle, but in this sentence it behaves like an adjective describing a condition.

Because antes is followed by de before an infinitive:

• antes de sair = before leaving / before I leave

This is the normal pattern in Portuguese:

• antes de dormir = before sleeping
• antes de comer = before eating
• antes de entrar = before entering

So antes sair is not correct in standard Portuguese.

The subject is understood from the context, and here it is the same person as eu in the main clause.

So:

• Eu preciso verificar... antes de sair.
• understood meaning: before I leave

Portuguese often leaves the subject unspoken when it is clear.

If you want to make the subject explicit, you can say:

• antes de eu sair = before I leave

That is also correct, but in this sentence the shorter antes de sair is more natural because the subject is obvious.

Yes, absolutely.

• Eu preciso verificar se a porta está fechada antes de sair.
• Eu preciso verificar se a porta está fechada antes de eu sair.

Both are correct.

The version without eu is usually smoother when the subject is the same as in the main clause. The version with eu adds explicitness or emphasis.

This is the normal Portuguese order for check whether/if...:

• verificar
  • se
    • clause

The embedded clause follows regular order:

• a porta = subject
• está = verb
• fechada = complement/adjective

So literally it is:

• verify if the door is closed

That structure is very common and natural.

Yes. A few common alternatives are:

• checar = to check
• conferir = to check, confirm
• ver = to see/check

Examples:

• Eu preciso checar se a porta está fechada.
• Eu preciso conferir se a porta está fechada.
• Eu preciso ver se a porta está fechada.

In everyday Brazilian Portuguese, ver se is especially common in casual speech. Verificar sounds a bit more formal or neutral.

The accent shows both pronunciation and stress.

• está is stressed on the last syllable: es-TÁ

It also helps distinguish the present form está from other related forms. Written accents are important in Portuguese, so you should include it.

Yes, and that is very natural in Brazilian Portuguese:

• Preciso verificar se a porta está fechada antes de sair.

Because preciso already tells us the subject is I, the pronoun is optional. In fact, omitting it is often the more natural choice unless you want emphasis:

• Eu preciso verificar... = maybe a little more emphatic
• Preciso verificar... = very natural, neutral