Most people get in touch for the first time with the gerundio in the construction estar + gerundio, that corresponds more or less to the English continuous form. The structure is the same as in Portuguese. We form it with the auxiliary verb to and the continuous form.
I am reading a book
brasilian portuguese: Eu estou lendo um livro.
european portuguese: Eu estou a ler um livro.
For reasons still to be discussed about it is not so often used in roman languages as in English, but basically the context it is used is the same. We use it if the fact that it is an ongoing action is the central aspect of the assertion. Together with adverbs like agora, now, there is a clear preference for the continuous form. If we put these two sentences, "O que fazes agora?" <=> "What do you do right now?" and "O que está fazendo agora?" <=> "What are you doing right now?" in google, in quotation marks, otherwise we get any page where these words are found in any order, we get the following results.
O que fazes agora?
O que está fazendo agora?
340 000 Treffer
The continuous form is highly recommended as well in the case that a special situation differs from normality.
Estou falando espanhol, mas falo brasileiro, eu sou do Brasil.
Right now I am speaking Spanish, but I speak Portuguese, I am from Brasil.
The meaning of "Ela fala espanhol" would mean that she knows Spanish. "Ela está falando espanhol" means that in concrete situation she speaks Spanish, what is not necessarily her mother tongue. The fact that it is an ongoing action is the central aspect as well in the following sentences.
Você está trabalhando o pode me ajudar?
Are you working or con you help me?
Fica quieto, estou falando com ele, não estou falando contigo.
Keep quite, I am talking with him and not with you.
It is often said that the construction estar + gerund in roman languages corresponds to the English continuous form what is true in the present, but not in the past. Roman languages have one verbal tense more than English and one of theses verb tenses is reserved to describe ongoing and persisten actions, the imperfeito. With the English simple past we can describe as well persistent states, events etc, "He worked for many years as an engineer, no clue what he is doing now", but can describe as well accomplished actions in an accomplished past, "He decided to go away". In other words, the simple past is not very specialised. If we want to describe an ongoing action that is interrupted by another action we use the continuous form in English: "I was sleeping, when he entered the room." The imperfeito of the roman languages is more specialised and can therefore be used as well to describe an action that is interrupted by another action. In Portuguese the sentence "I was walking down the street when suddenly a car stopped" can be translated without any difference in meaning with the imperfeito or with the construction estar + gerundio.
Estava passeando pela rua quando vi o carro carro parar de repente.
Passeava pela rua quando vi o carro parar de repente.
If we put the relevant part in google, we see a difference. There is some preference for the estar + gerundio construction, but the difference is not really relevant, well possible that with other examples we get a different result.
Estava passeando pela rua quando ...
Passeava pela rua quando...
If we do the same thing with the corresponding English sentences the result is very clear.
I walked down the street when
I was walking down the street when
119 000 Treffer.
It is possible that there are contexts where the construction estar + gerundio is preferred as well in the past. The imperfeito have a lot of secondary meanings, can describe two parallely running actions, actions that are continuously repeated in the past, actions or events whose beginning and end is irrelevant or unknown. It is possible that there are contexts where all these secondary meaning has to be excluded. However in most cases the estar + gerundion construction in the past is interchangeable with the imperfeito.
In European Portuguese the structure that corresponds to estar + gerundio is estar + a + infinitivo. Both constructions are completely equivalent.
Quando entrei, ela estava a dormir.
When I came in, she was sleeping.
Ele está a fazer o que pode para resolver o problema.
He is doing everything he can to resolve the problem.