Se fizer mais frio, vou usar o edredão grosso que a minha avó me deu.

Because after se when you are talking about a future possibility, Portuguese uses the future subjunctive.

So:

• Se fizer mais frio = If it gets colder / if it becomes colder
• Se faz mais frio would sound wrong for this future meaning

Compare:

• Se fizer mais frio amanhã, vou usar... = future possibility
• Se faz frio no inverno, fico em casa. = not natural in this sense; for general situations Portuguese would usually phrase it differently, such as Quando faz frio...

This is one of the most important patterns in Portuguese:

• se + future subjunctive
• main clause often with future or near future: vou usar

Fizer is the future subjunctive of fazer.

Fazer is irregular, so this form is not built in an obvious way from the present tense. The future subjunctive is based on the preterite stem:

• ele fez = he/she did
• from that stem you get se eu fizer, se tu fizeres, se ele fizer, etc.

A few forms:

• se eu fizer
• se tu fizeres
• se ele/ela fizer
• se nós fizermos
• se vós fizerdes
• se eles/elas fizerem

So fizer here means does / makes / becomes in a future conditional context.

In Portuguese, weather is often expressed with fazer:

• faz frio = it is cold
• faz calor = it is hot
• faz sol = it is sunny

So Se fizer mais frio... literally uses fazer, but in natural English it is usually translated as:

• If it gets colder
• If it’s colder

This is just a normal Portuguese weather expression.

Mais frio means colder.

Portuguese usually forms the comparative with:

• mais + adjective

So:

• frio = cold
• mais frio = colder

Unlike English, Portuguese usually does not make a special one-word comparative form like colder. It simply says more cold.

Examples:

• mais quente = warmer / hotter
• mais alto = taller
• mais caro = more expensive

Both are possible, but vou usar is much more common in everyday speech.

• vou usar = near future / going to use
• usarei = simple future / I will use

In European Portuguese, as in English, the near future with ir + infinitive is very common and often sounds more natural in conversation.

So:

• Se fizer mais frio, vou usar... = very natural spoken Portuguese
• Se fizer mais frio, usarei... = correct, but more formal or written

Portuguese uses definite articles much more often than English.

So o edredão grosso literally means the thick duvet.

In this sentence, it refers to a specific duvet: the thick one that the speaker’s grandmother gave them.

That is why o is used:

• um edredão grosso = a thick duvet
• o edredão grosso = the thick duvet

In Portuguese, adjectives often come after the noun.

So:

• edredão grosso = thick duvet

That is the most neutral and normal order here.

Also, grosso in this context means thick rather than fat. For objects like blankets, coats, or books, grosso often refers to thickness.

Examples:

• um casaco grosso = a thick coat
• um livro grosso = a thick book

This is a relative clause modifying o edredão grosso.

It means:

• que = that / which
• a minha avó = my grandmother
• me deu = gave me

So the whole part means:

• that my grandmother gave me

The word que connects the noun to extra information about it:

• o edredão grosso que a minha avó me deu
• the thick duvet that my grandmother gave me

Because in this clause, que triggers proclisis, which means the object pronoun comes before the verb.

So standard Portuguese is:

• que a minha avó me deu

not:

• que a minha avó deu-me

The pronoun me means to me here:

• dar algo a alguém = to give something to someone
• deu-me / me deu = gave me

In European Portuguese, pronoun position can be tricky, but one important rule is that words like que usually force the pronoun to come before the verb.

In European Portuguese, it is very common to use the definite article before possessives:

• a minha avó = my grandmother
• o meu pai = my father
• a nossa casa = our house

So a minha avó is the normal European Portuguese phrasing.

You may sometimes see the article omitted, especially in other varieties or special contexts, but for Portugal Portuguese, a minha avó is exactly what a learner should expect to hear.

Deu is the 3rd person singular preterite of dar:

• dar = to give
• ele/ela deu = he/she gave

It is irregular, so it does not look much like the infinitive.

Here it is used because the grandmother’s act of giving the duvet is seen as a completed past action.

Yes, edredão is the normal word in European Portuguese for a duvet or quilted comforter.

A learner may notice a spelling difference in Brazilian Portuguese:

• Portugal: edredão
• Brazil: edredom

They refer to the same kind of thing, but the Portugal form in your sentence is edredão.

The accent changes both the pronunciation and the meaning:

• avó = grandmother
• avô = grandfather

So in this sentence, a minha avó clearly means my grandmother.

This is a very common pair for learners to mix up, so it is worth memorising early.