Minhas chaves costumam sumir quando estou com pressa.

Because chave is a feminine noun in Portuguese: a chave. Its plural is as chaves.

So the possessive has to agree with the noun:

• minha chave = my key
• minhas chaves = my keys

Even though the owner is not feminine, the possessive agrees with the thing possessed, not with the speaker.

Because the subject is minhas chaves, which is third person plural.

The verb costumar has to agree with the subject:

• minha chave costuma sumir
• minhas chaves costumam sumir

So costumam matches chaves.

Costumar + infinitive means to usually do something, to tend to do something, or to be in the habit of doing something.

So:

• costumam sumir = usually disappear, tend to disappear

It adds the idea of a repeated or typical pattern, not just a single event.

Both can work, but the nuance is a little different.

• Minhas chaves somem quando estou com pressa.
  = My keys disappear when I’m in a hurry.

• Minhas chaves costumam sumir quando estou com pressa.
  = My keys tend to disappear / usually disappear when I’m in a hurry.

Using costumam makes the habitual idea more explicit. It sounds a bit like this is something that often happens.

Both can mean to disappear, but sumir is very common in everyday Brazilian Portuguese and often sounds more natural in casual speech.

• sumir = very common, conversational
• desaparecer = also correct, sometimes a bit more neutral or formal depending on context

For misplaced objects like keys, sumir is extremely natural:

• Meu celular sumiu.
• As chaves sumiram.

It does not necessarily mean they literally vanished; it can also mean they are suddenly nowhere to be found.

Because after costumar, Portuguese uses the infinitive.

Pattern:

• costumar + infinitive

Examples:

• Eu costumo estudar à noite.
• Ela costuma chegar cedo.
• As chaves costumam sumir.

So costumam sumir is correct, not costumam somem.

Portuguese often leaves out subject pronouns when the verb form already makes the subject clear.

• estou already tells you it means I am
• so quando estou com pressa naturally means when I’m in a hurry

You could say:

• quando eu estou com pressa

But in many cases, especially in Brazilian Portuguese, the eu is optional here unless you want emphasis or clarity.

Because estar com pressa is the normal expression for to be in a hurry.

• estar com pressa = to be in a hurry
• ser apressado = to be a hasty/impatient/rushed kind of person

So:

• Estou com pressa = I’m in a hurry right now
• Sou apressado = I’m a hasty person / I tend to rush

In your sentence, the speaker means a temporary situation, so estou com pressa is the natural choice.

Literally, it is with hurry, but the natural English meaning is in a hurry.

This is a fixed expression in Portuguese:

• Estou com pressa. = I’m in a hurry.
• Ela saiu com pressa. = She left in a hurry.

It is one of those expressions that should be learned as a chunk.

In Portuguese, the present tense is often used for habits, routines, and things that generally happen.

So this sentence is not about one specific moment. It means this is a recurring pattern:

• Minhas chaves costumam sumir quando estou com pressa.

The present tense here works like English usually disappear or tend to disappear.

Yes. Both are correct in Brazilian Portuguese:

• Minhas chaves costumam sumir...
• As minhas chaves costumam sumir...

In Brazilian Portuguese, possessives can appear with or without the definite article. The version without the article is very common and natural.

The choice often depends on region, style, and rhythm, not on a big difference in meaning.

Yes, in everyday use it often suggests that something has gone missing or is nowhere to be found, not necessarily that it literally vanished.

So Minhas chaves costumam sumir can imply:

• I keep misplacing them
• they seem to disappear on me
• I can never find them at those moments

It has a slightly informal, everyday feel that fits this kind of situation very well.

The given order is very natural:

• Minhas chaves costumam sumir quando estou com pressa.

But Portuguese allows some flexibility. For example:

• Quando estou com pressa, minhas chaves costumam sumir.

This version puts more focus on the time/situation when I’m in a hurry.

Both are correct. The original version sounds very natural in conversation.

The sentence already sounds natural. In speech, you might also hear slight variations like:

• Minhas chaves sempre somem quando tô com pressa.
• As minhas chaves costumam sumir quando eu tô com pressa.

Notes:

• tô is the spoken contraction of estou
• sempre somem is a very common alternative to costumam sumir

But the original sentence is completely natural and correct in standard Brazilian Portuguese.