Quando eu espirro muito, meu nariz fica vermelho.

Quando means when. It introduces a time clause:

Quando eu espirro muito = When I sneeze a lot

So the sentence is structured like this:

• Quando eu espirro muito = when I sneeze a lot
• meu nariz fica vermelho = my nose gets/becomes red

Portuguese often uses quando the same way English uses when in sentences about repeated or general situations.

Yes, you could say Quando espirro muito.

In Portuguese, subject pronouns are often omitted because the verb ending already shows who the subject is:

• espirro = I sneeze

So both are natural:

• Quando eu espirro muito...
• Quando espirro muito...

Including eu can add clarity or emphasis, but it is not required here.

Here, espirro is the 1st person singular present tense of the verb espirrar:

• espirrar = to sneeze
• eu espirro = I sneeze

So:

• Quando eu espirro muito = when I sneeze a lot

A learner may notice that espirro can also be a noun in Portuguese:

• um espirro = a sneeze

But in this sentence, it is clearly a verb, because it follows eu.

Because muito here means a lot, modifying the action of sneezing:

• espirro muito = I sneeze a lot

This is a very common placement in Portuguese: the word showing degree or frequency often comes after the verb.

Compare:

• Eu como muito = I eat a lot
• Eu trabalho muito = I work a lot
• Eu espirro muito = I sneeze a lot

Both are possible, but meu nariz is very natural in Brazilian Portuguese.

Portuguese possessives can appear:

• with an article: o meu nariz
• without an article: meu nariz

In Brazil, dropping the article is very common in many situations, especially in everyday speech.

So these both can work:

• meu nariz fica vermelho
• o meu nariz fica vermelho

The version without the article sounds simple and natural.

This is an important difference.

• ser (é) is usually for identity, permanent characteristics, or definitions
• ficar often means to become, to get, or to end up

So:

• meu nariz é vermelho = my nose is red
  This sounds more like a general characteristic.
• meu nariz fica vermelho = my nose gets red / becomes red
  This matches the idea of a change caused by sneezing.

In this sentence, the nose is not always red. It turns red after sneezing a lot, so fica is the natural choice.

Fica is the 3rd person singular present tense of ficar:

• ficar = to become / to get / to stay
• ele/ela fica = he/she gets/becomes/stays

Here the subject is meu nariz, which is singular, so the verb is fica:

• meu nariz fica vermelho

Portuguese often uses the present tense for general truths or habitual situations, just like English:

• Quando eu espirro muito, meu nariz fica vermelho.
• When I sneeze a lot, my nose gets red.

Because vermelho must agree with nariz, and nariz is a masculine singular noun.

Agreement in Portuguese means adjectives usually match the noun in gender and number:

• nariz vermelho = red nose
• camisa vermelha = red shirt
• olhos vermelhos = red eyes

So in this sentence:

• meu nariz = masculine singular
• therefore vermelho = masculine singular

Yes, it is grammatically possible, but it is less natural in everyday Brazilian Portuguese for this context.

• ficar vermelho = the normal, idiomatic way to say get red / turn red
• se tornar vermelho = become red, but it sounds more formal or less conversational here

A Brazilian speaker would usually prefer:

• meu nariz fica vermelho

The comma separates the introductory quando clause from the main clause:

• Quando eu espirro muito, = When I sneeze a lot,
• meu nariz fica vermelho. = my nose gets red.

This is also common in English:

• When I sneeze a lot, my nose gets red.

The comma helps show the sentence has two parts.

Yes. You can put the main clause first:

• Meu nariz fica vermelho quando eu espirro muito.

This means the same thing:

• My nose gets red when I sneeze a lot.

Both word orders are correct. Starting with quando puts the time condition first, while starting with meu nariz puts focus first on what happens.

A simple pronunciation guide is:

KWAN-doo eh-oo es-PEE-hoh MOO-ee-too, meh-oo nah-REEZ FEE-kah veh-HEL-yoo

A few useful notes:

• quando: the qu sounds like kw
• eu often sounds like a quick eh-oo
• espirro: the rr here is not the strong guttural rr seen in words like carro; in espirro, it is a single r sound because of where it appears
• nariz is stressed on the last syllable: na-RIZ
• fica sounds like FEE-kah
• vermelho has the lh sound, similar to ly in million, though not exactly the same

It describes a general/habitual situation.

The present tense in Portuguese often expresses something that usually happens:

• Quando eu espirro muito, meu nariz fica vermelho.
• When I sneeze a lot, my nose gets red.

This does not necessarily mean it is happening right now. It means this is what generally happens in that situation.

If you wanted to describe something happening right now, you would normally need more context, such as:

• Estou espirrando muito, e meu nariz está ficando vermelho.
• I’m sneezing a lot, and my nose is getting red.