Minha mãe costuma cozinhar espinafre com alho para o jantar.

Costuma comes from the verb costumar, which is used to talk about a habit or something that usually happens.

So Minha mãe costuma cozinhar... means something like:

• My mother usually cooks...
• My mother tends to cook...
• My mother habitually cooks...

It does not mean she is cooking right now. It describes what she normally does.

After costumar, Portuguese uses another verb in the infinitive form.

So:

• costuma cozinhar = usually cooks
• literally: is accustomed to cook / tends to cook

This is very common in Portuguese:

• Eu costumo estudar à noite. = I usually study at night.
• Eles costumam sair cedo. = They usually leave early.

So cozinhar stays in the infinitive because it follows costuma.

Because mãe is a feminine noun.

In Portuguese, possessives agree with the thing possessed, not with the owner. Since mãe is feminine singular, you use minha:

• minha mãe = my mother
• meu pai = my father

Compare:

• meu livro = my book
• minha casa = my house

Yes. In Brazilian Portuguese, both minha mãe and a minha mãe are possible.

So these both work:

• Minha mãe costuma cozinhar espinafre com alho para o jantar.
• A minha mãe costuma cozinhar espinafre com alho para o jantar.

In Brazil, using the article before possessives is very common, but omitting it is also normal, especially in simpler or more neutral sentences.

The ã shows a nasal sound, which is very important in Portuguese pronunciation.

Mãe is pronounced roughly like mãyn with a nasal vowel, not like a plain mae.

A few helpful points:

• ã is nasal
• mãe is one syllable for many speakers, or very close to it
• it sounds somewhat like the my in English, but nasalized

The accent mark is not optional here; it is part of the correct spelling.

Costuma is:

• 3rd person singular
• present tense
• from costumar

It matches minha mãe = she / my mother

A quick pattern:

• eu costumo = I usually
• você / ele / ela costuma = you/he/she usually
• nós costumamos = we usually
• eles / elas costumam = they usually

So Minha mãe costuma means My mother usually...

Portuguese often does not need an article when talking about food or ingredients in a general way.

So:

• cozinhar espinafre com alho = cook spinach with garlic

This sounds natural when you are talking about the dish or ingredient generally, not a specific spinach or a specific garlic.

You could sometimes hear articles in other contexts, but here the article-free version is the most natural.

Here com alho describes how the spinach is prepared: it means spinach with garlic.

So the structure is understood as:

• cozinhar espinafre com alho = cook spinach with garlic

In other words, com alho goes with espinafre / the dish, not with minha mãe.

Context makes this very clear.

Para o jantar means for dinner — in other words, the spinach is being prepared to serve at dinner.

• para o jantar = for dinner

If you say no jantar, that usually means at dinner or during dinner, which is a different idea.

So:

• Ela cozinha espinafre para o jantar. = She cooks spinach for dinner.
• Ela falou disso no jantar. = She talked about that at dinner.

Jantar can be a noun or a verb.

Here it is a noun meaning dinner, so it takes the article o:

• o jantar = dinner / the dinner meal

As a verb, jantar means to have dinner / to dine:

• Nós vamos jantar agora. = We’re going to have dinner now.

In your sentence, jantar is definitely the noun.

Yes. In spoken Brazilian Portuguese, para o is very often shortened to pro.

So these are equivalent:

• para o jantar
• pro jantar

The full form para o jantar is a bit more neutral or careful.
Pro jantar is extremely common in everyday speech.

The sentence follows a very normal Portuguese pattern:

• Minha mãe = subject
• costuma cozinhar = verb phrase
• espinafre com alho = object / food being prepared
• para o jantar = purpose or occasion

So it is structured like:

[Subject] + [usually + verb] + [thing] + [for dinner]

This order sounds natural and straightforward in Portuguese.

Not exactly. Costuma cozinhar is about habit, not preference.

• costuma cozinhar = usually cooks
• gosta de cozinhar = likes to cook

So:

• Minha mãe costuma cozinhar espinafre com alho para o jantar.
  = My mother usually cooks spinach with garlic for dinner.

If you want likes to cook, you would say:

• Minha mãe gosta de cozinhar espinafre com alho para o jantar.

Cozinhar is a very natural choice here. It means to cook.

You might also hear other verbs depending on context:

• fazer = to make
• preparar = to prepare

Examples:

• Minha mãe costuma fazer espinafre com alho para o jantar.
• Minha mãe costuma preparar espinafre com alho para o jantar.

But cozinhar is perfectly normal and clear.

Yes, it works naturally in Brazilian Portuguese, and it would also be understood in European Portuguese.

A few things that feel especially natural in Brazil:

• using costumar for habits
• saying para o jantar or pro jantar
• optionally omitting the article before minha mãe

So the sentence is fully good Brazilian Portuguese.