Compramos un disfraz.

Yes. Compramos can mean either:

• we bought (preterite / completed past action), or
• we buy (present tense)

This happens because the nosotros form of many -ar verbs looks the same in the present and the preterite.

So:

• compramos un disfraz = we bought a costume
• compramos disfraces aquí = we buy costumes here

Usually, context tells you which meaning is intended. If this sentence is being presented as a past action, then it means we bought a costume.

It comes from the verb comprar, which means to buy.

Here is a quick breakdown:

• comprar = to buy
• compré = I bought
• compraste = you bought
• compró = he/she/it bought
• compramos = we bought
• compraron = they bought

So compramos is the we form.

Because Spanish often leaves out subject pronouns when they are not needed.

In English, you must say we bought.
In Spanish, the verb ending already shows the subject:

• compramos = we bought / we buy

So nosotros compramos un disfraz is possible, but usually just compramos un disfraz sounds more natural unless you want emphasis or contrast.

For example:

• Nosotros compramos un disfraz, pero ellos no.
  We bought a costume, but they didn’t.

Un means a or an.

So:

• un disfraz = a costume

It is the masculine singular indefinite article.

Spanish indefinite articles are:

• un = a/an (masculine singular)
• una = a/an (feminine singular)
• unos = some (masculine plural)
• unas = some (feminine plural)

Since disfraz is a masculine singular noun, it uses un.

Because disfraz is a masculine noun in Spanish.

In Spanish, nouns have grammatical gender, so the article must match the noun:

• un disfraz
• not una disfraz

This gender is grammatical, not necessarily logical. You usually just learn the noun together with its article:

• el disfraz = the costume
• un disfraz = a costume

Disfraz means costume or fancy-dress outfit.

In Spain, it is very commonly used for the kind of costume you wear at:

• Carnival
• Halloween
• a themed party
• school dress-up events

So un disfraz is not a disguise in the secret-agent sense most of the time. It usually means a costume you wear for fun or celebration.

A simple pronunciation guide is:

kom-PRA-mos oon dees-FRATH
in much of Spain

In many parts of Latin America, the last word sounds more like:

dees-FRAS

A few notes:

• co- sounds like koh
• pra has the stress: com-PRA-mos
• un is pronounced like oon
• in most of Spain, z is pronounced like the th in think, so disfraz sounds like dees-FRATH

Because the sentence says one costume, not costumes.

• un disfraz = a costume
• unos disfraces = some costumes

The verb compramos tells you who did the action (we), not how many items were bought. The noun itself shows the number of the thing bought.

So:

• Compramos un disfraz. = We bought a costume.
• Compramos unos disfraces. = We bought some costumes.

Yes, it is grammatically correct.

• Compramos un disfraz.
• Nosotros compramos un disfraz.

Both are correct. The version without nosotros is usually more natural in everyday Spanish because the verb already shows the subject.

You would add nosotros mainly for:

• emphasis
• contrast
• clarity

For example:

• Nosotros compramos un disfraz; ellos compraron dos.
  We bought one costume; they bought two.

Normally, no.

Compramos by itself usually means:

• we buy, or
• we bought

If you want to say we are buying, Spanish more commonly uses:

• Estamos comprando un disfraz.

That is the present progressive, literally we are buying a costume.

So:

• Compramos un disfraz. = We buy / We bought a costume.
• Estamos comprando un disfraz. = We are buying a costume.

You put no before the verb:

• No compramos un disfraz.

Depending on context, that can mean:

• We didn’t buy a costume, or
• We don’t buy a costume

If you want to be very clear in the past, context usually does the job, but Spanish speakers often rely on the surrounding sentence or situation rather than changing this form.