Preciso transmitir a declaração do Imposto de Renda hoje.

Because Portuguese often drops the subject pronoun when the verb form already shows who the subject is.

• Preciso is the I form of precisar
• So Preciso transmitir... already means I need to submit/file...

You can say Eu preciso transmitir... if you want emphasis, contrast, or extra clarity, but leaving out eu is very normal.

Here, preciso is the verb precisar in the I form, so it means I need.

This can confuse English speakers because preciso can also be an adjective meaning precise or exact in other contexts.

In this sentence, it is clearly a verb because it is followed by another verb, transmitir:

• Preciso transmitir = I need to submit/transmit

Because Portuguese uses two common patterns with precisar:

• precisar + infinitive = to need to do something
• precisar de + noun = to need something

So:

• Preciso transmitir a declaração. = I need to submit the return.
• Preciso de ajuda. = I need help.

That is why there is no de before transmitir.

In tax language, transmitir usually means to submit electronically.

So this is not the everyday meaning of transmit in the sense of broadcasting a signal. In the context of taxes in Brazil, transmitir a declaração is a standard official way to say that you are sending your tax return to the government system.

A few close English equivalents would be:

• to file
• to submit
• to send electronically

Portuguese uses articles more often than English does.

Here, a declaração means the return / the declaration, referring to a specific, understood document: the tax return the speaker has to submit.

In English, we might sometimes say just I need to file my taxes today, but Portuguese often keeps the article with the noun:

• a declaração = the return
• a declaração do Imposto de Renda = the income tax return

Do is a contraction of:

• de + o = do

So:

• a declaração do Imposto de Renda literally means
• the declaration of the income tax

In more natural English, that becomes:

• the income tax return
• the income tax declaration

So do here means of the.

Because it is being treated like the official name of the tax.

In Brazilian usage, you may see both:

• Imposto de Renda
• imposto de renda

Capital letters are common when people are referring to the official tax category almost like a proper name, especially in formal, bureaucratic, or instructional writing.

In casual writing, lowercase is also very common.

No. Hoje can move around, although some positions sound more natural than others.

Natural options include:

• Preciso transmitir a declaração do Imposto de Renda hoje.
• Hoje preciso transmitir a declaração do Imposto de Renda.

Both are good. Putting hoje at the end is very neutral and common.

A version like Preciso hoje transmitir... is possible, but it sounds less natural in everyday speech.

It is very natural in an official or tax-related context, especially when you mean submit the completed return electronically.

But in everyday speech, Brazilians might also say:

• Preciso enviar a declaração do Imposto de Renda hoje.
• Preciso entregar a declaração do Imposto de Renda hoje.
• Preciso declarar o Imposto de Renda hoje.

The nuance changes a little:

• transmitir = submit electronically, somewhat technical/official
• enviar = send
• entregar = hand in / submit
• declarar o Imposto de Renda = do/file your taxes more generally

Yes, and that is extremely common.

But the meaning is slightly broader:

• declarar o Imposto de Renda = to do/file your income tax
• transmitir a declaração do Imposto de Renda = to actually submit the tax return, especially online

So if you want to be specific about the final act of sending the return, transmitir a declaração is more precise.

The sentence is built like this:

• [implied subject] = I
• Preciso = need
• transmitir = to submit/transmit
• a declaração do Imposto de Renda = the income tax return
• hoje = today

So the pattern is:

• precisar + infinitive + object + time expression

This is a very useful structure to learn. For example:

• Preciso estudar hoje. = I need to study today.
• Preciso mandar esse e-mail agora. = I need to send this email now.

A Brazilian would definitely understand it. It sounds normal, but a bit formal or administrative because of transmitir and declaração do Imposto de Renda.

That makes sense, since taxes are often talked about in formal language.

A more conversational version might be:

• Preciso enviar minha declaração do Imposto de Renda hoje.
• Preciso declarar meu Imposto de Renda hoje.

So your sentence is correct and natural, especially in a bureaucratic or official context.