Ontem também estava nublado, e mesmo assim nós fomos caminhar.

Because estava uses the imperfect past, which is very common for background description in the past.

In this sentence, the weather is being described as the situation in the background:

• Ontem também estava nublado = Yesterday it was cloudy too

That is different from a completed event. Weather, time, atmosphere, and ongoing conditions often use the imperfect:

• Estava frio. = It was cold.
• Estava chovendo. = It was raining.
• Era tarde. = It was late.

Using foi would sound much less natural here, because foi usually presents something as a completed, bounded fact rather than a background condition.

Também means also / too.

So:

• Ontem também estava nublado = Yesterday it was cloudy too / also cloudy yesterday

It connects this sentence to some previous idea, such as:

• Today is cloudy.
• Ontem também estava nublado.

In Portuguese, também is flexible in position, but here it naturally comes after ontem.

Mesmo assim means even so, even then, all the same, or nevertheless.

In this sentence:

• ...e mesmo assim nós fomos caminhar.
• ...and even so, we went for a walk.

It shows contrast:

• the weather was cloudy
• but despite that, they still went walking

This is a very useful expression in Portuguese.

Examples:

• Estava cansado, mas mesmo assim foi trabalhar. = He was tired, but even so he went to work.
• Chovia, e mesmo assim saímos. = It was raining, and even so we went out.

Yes, it could be omitted.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear. So both are correct:

• ...mesmo assim nós fomos caminhar.
• ...mesmo assim fomos caminhar.

Because fomos clearly means we went, the pronoun is not necessary.

Why include nós, then?

Common reasons:

• emphasis
• clarity
• contrast with someone else
• a slightly more explicit style

In everyday Brazilian Portuguese, people also very often say:

• ...mesmo assim a gente foi caminhar.

That is extremely common in speech.

Both are possible, but they are not exactly the same.

fomos caminhar

Literally, this is like we went to walk or more naturally we went for a walk / went walking.

It emphasizes:

• the idea of going out
• the purpose of going somewhere in order to walk

caminhamos

This simply means we walked.

It emphasizes the action itself, not so much the idea of setting out to do it.

So:

• Nós fomos caminhar. = We went for a walk.
• Nós caminhamos. = We walked.

In this sentence, fomos caminhar sounds very natural because it suggests they decided to go out walking despite the cloudy weather.

Here, fomos is from ir.

Why?

Because it is followed by an infinitive:

• fomos caminhar

That pattern is very common:

• fui comprar pão = I went to buy bread
• fomos almoçar = we went to have lunch
• eles foram nadar = they went swimming

It is true that fomos can also be the past form of ser:

• Nós fomos felizes naquela época. = We were happy in that period.

So context tells you which verb it is. In fomos caminhar, it is definitely ir.

Caminhar means to walk, especially as an activity such as walking for exercise, walking outdoors, or going for a walk.

Andar can also mean to walk, but it is broader and can also mean:

• to move around
• to go
• to function
• to ride/travel in some contexts

Examples:

• Gosto de caminhar no parque. = I like walking in the park.
• Ele anda muito rápido. = He walks very fast.
• O relógio não anda. = The clock does not work.
• Andar de ônibus = to ride/go by bus

In this sentence, caminhar is a good choice because it sounds like going for a walk.

Because this is an impersonal weather expression.

In estava nublado, there is no real personal subject like o dia or a manhã stated explicitly. Portuguese uses the masculine singular by default in many impersonal expressions of this kind.

So you commonly hear:

• Está nublado. = It’s cloudy.
• Está ensolarado. = It’s sunny.

If you explicitly name a feminine noun, then agreement changes:

• A manhã estava nublada. = The morning was cloudy.
• A tarde estava nublada. = The afternoon was cloudy.

But without an explicit noun, nublado is the normal form.

Yes. Ontem is flexible.

The given version:

• Ontem também estava nublado...

is very natural.

But you could also say:

• Também estava nublado ontem...
• Estava nublado ontem também...

These are all understandable, but the placement changes what feels most prominent.

In the original sentence

• Ontem comes first, so the time is established immediately.
• Then também adds the idea of too / also.

That word order is very natural when you are comparing yesterday with another day already being discussed.

Yes, often you can.

Here are the two versions:

• Ontem também estava nublado, e mesmo assim nós fomos caminhar.
• Ontem também estava nublado, mas mesmo assim nós fomos caminhar.

Both are natural.

Difference

• e mesmo assim = literally and even so
• mas mesmo assim = but even so

Mas mesmo assim makes the contrast slightly more explicit.
E mesmo assim is also very common and sounds perfectly natural.

Yes, it is natural. But in casual Brazilian Portuguese, many speakers would choose a slightly more conversational version, especially with a gente instead of nós:

• Ontem também estava nublado, e mesmo assim a gente foi caminhar.

That is extremely common in speech.

Other natural variations:

• Ontem também estava nublado, mas mesmo assim a gente foi caminhar.
• Ontem também estava nublado, e mesmo assim fomos caminhar.

So the original sentence is correct and natural, but in everyday spoken Brazilian Portuguese, a gente foi is often more common than nós fomos.

It has two main parts joined together:

1. Background situation

• Ontem também estava nublado
• Yesterday it was cloudy too

This sets the scene.

2. Main action despite that situation

• e mesmo assim nós fomos caminhar
• and even so we went for a walk

So the logic is:

• condition/background: it was cloudy
• unexpected result: they went walking anyway

This kind of structure is very common in Portuguese:

• Estava cansado, mas mesmo assim saiu.
• Era tarde, e mesmo assim continuaram trabalhando.

Here are a few useful comparisons:

Original

• Ontem também estava nublado, e mesmo assim nós fomos caminhar.
• Yesterday it was cloudy too, and even so we went for a walk.

Without também

• Ontem estava nublado, e mesmo assim nós fomos caminhar.
• Yesterday it was cloudy, and even so we went for a walk.

This loses the idea of too / also.

Without mesmo assim

• Ontem também estava nublado, e nós fomos caminhar.
• Yesterday it was cloudy too, and we went for a walk.

This loses the sense of contrast or surprise.

Without nós

• Ontem também estava nublado, e mesmo assim fomos caminhar.
• Same meaning, just less explicit.

So each part contributes something:

• também = also
• mesmo assim = even so / nevertheless
• nós = explicit subject, optional