Minha mãe costuma cozinhar abobrinha com alho para o jantar.

Because mãe is a feminine noun in Portuguese, so the possessive has to agree with it:

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

So minha is the feminine form of my, and meu is the masculine form.

Yes. In Brazilian Portuguese, both are possible:

• Minha mãe costuma cozinhar...
• A minha mãe costuma cozinhar...

In Brazil, using the article before a possessive is very common, but leaving it out is also completely natural, especially in simple statements like this one.

Costuma comes from the verb costumar, which means to usually do something, to tend to, or to have the habit of.

So:

• Minha mãe cozinha... = My mother cooks... / My mother is cooking... depending on context
• Minha mãe costuma cozinhar... = My mother usually cooks... / My mother tends to cook...

It adds the idea of habit or custom.

Because after costumar, Portuguese uses another verb in the infinitive.

So the pattern is:

• costumar + infinitive

Examples:

• Eu costumo estudar à noite. = I usually study at night.
• Ela costuma correr de manhã. = She usually runs in the morning.

That is why you get:

• costuma cozinhar

not

• costuma cozinha

In Brazilian Portuguese, abobrinha usually means zucchini.

It looks like a diminutive because of -inha, but in this case it is simply the normal everyday word for the vegetable. Native speakers do not usually think of it as little pumpkin or anything literal like that.

A learner may also notice:

• abóbora = pumpkin / squash
• abobrinha = zucchini

Portuguese often leaves out the article when talking about food or ingredients in a general sense.

So:

• cozinhar abobrinha com alho = to cook zucchini with garlic

This sounds natural when describing what someone usually makes.

You could sometimes hear:

• cozinhar a abobrinha

but that would sound more like a specific zucchini already known in the context.

In this sentence, abobrinha is understood more generally as the dish or ingredient.

Com means with, so:

• abobrinha com alho = zucchini with garlic

This is just the normal way to say that one ingredient is prepared together with another.

For example:

• arroz com feijão = rice with beans
• pão com manteiga = bread with butter

Para o jantar means for dinner, emphasizing purpose:

• She cooks it for dinner.

No jantar means at dinner or during dinner, which is more about the time or occasion itself.

So the difference is roughly:

• para o jantar = for dinner, to be eaten at dinner
• no jantar = at dinner

In this sentence, para o jantar is the most natural choice because it tells you what meal the food is intended for.

Here, jantar means dinner as a noun.

That is why it appears in:

• para o jantar = for dinner

But jantar can also be a verb:

• jantar = to have dinner / to dine

Examples:

• O jantar está pronto. = Dinner is ready.
• Vamos jantar às oito. = We’re going to have dinner at eight.

So it can be either a noun or a verb, depending on the structure.

Yes, it is very normal. The structure is:

• Minha mãe = subject
• costuma cozinhar = verb phrase
• abobrinha com alho = object / what she cooks
• para o jantar = purpose / meal context

So it follows a very common Portuguese pattern:

Subject + verb + object + extra information

That makes this sentence straightforward and natural.

Mãe is pronounced roughly like my in English, but with a nasal sound.

Important points:

• ã is nasal
• e at the end helps form the diphthong

A rough approximation is maing without fully pronouncing the final ng.

The tilde ~ is important because it marks nasalization:

• mae would not be the same as mãe

Yes. That is a very good translation.

Because of costuma, the sentence can be understood as:

• My mother usually cooks zucchini with garlic for dinner
• My mother tends to cook zucchini with garlic for dinner
• My mother has the habit of cooking zucchini with garlic for dinner

All of these capture the habitual meaning well.