Learn Hardcore Portuguese (Portugal): Se estiveres a espirrar muito, usa um lenço de papel em vez da manga. - If you are sneezing a lot, use a tissue instead of your sleeve.

Estiveres is the present subjunctive (2nd person singular, tu). In Portuguese, after se meaning if (introducing a condition that may or may not happen), you normally use the subjunctive, not the indicative:

Se estiveres doente, fica em casa. – If you’re (might be) sick, stay at home.
Se estás doente, … – grammatically possible, but sounds like the speaker is assuming it’s true and asking for confirmation, more like “If you are sick (and I believe you are)…”

In your sentence, the idea is a possible condition, so se estiveres is the natural choice.

What exactly does estiveres a espirrar express compared with just espirrares?

Both are possible, but the nuance is slightly different:

Se estiveres a espirrar muito – If you are sneezing a lot (at that moment / in that situation)
- Continuous or ongoing action, similar to English “if you’re sneezing a lot.”
Se espirrares muito – If you sneeze a lot (in general / whenever that happens)
- More like a repeated or habitual action.

In everyday speech, estar a + infinitive is very common in European Portuguese to express an ongoing action.

Why is it a espirrar and not something like espirrando?

European Portuguese generally doesn’t use the -ndo form (the gerund) for continuous actions the way English or Brazilian Portuguese does.

Instead, it uses:

estar a + infinitive
- estou a espirrar – I am sneezing
- estiveres a espirrar – you are sneezing (in a conditional context here)

So a espirrar is just part of the structure estar a + espirrar to indicate a progressive action.

Why is the verb usa and not usas?

Usa here is the imperative form for tu (2nd person singular):

usar → imperative (tu): usa
- Usa um lenço de papel. – Use a tissue.

The form usas is the present indicative (you use / you are using), not the imperative. In the affirmative imperative for tu, you usually take the 3rd person singular of the present indicative:

falar: ele fala → fala (tu) – speak
comer: ele come → come (tu) – eat
usar: ele usa → usa (tu) – use

Is this sentence talking to tu or você? How do I know?

The sentence is clearly talking to tu (informal you) because:

estiveres – subjunctive form for tu
usa – imperative form for tu

If it were addressing você (more formal or distant), it would be:

Se estiver a espirrar muito, use um lenço de papel em vez da manga.

So the original is informal / familiar.

What does lenço de papel mean exactly? Is it different from guardanapo?

lenço de papel – literally paper handkerchief; in practice this is what we call a tissue (like Kleenex) used for blowing your nose, wiping sneezes, etc.
guardanapo – a napkin, normally used at the table when eating.

In this context, “Use a tissue instead of your sleeve.” → usa um lenço de papel em vez da manga.

Why is it um lenço de papel and not o lenço de papel?

Here um (a / one) is used because we don’t mean a specific, previously mentioned tissue; we mean any tissue:

Usa um lenço de papel – Use a tissue (any tissue).
Usa o lenço de papel – Use the tissue (a particular one we both know about).

The indefinite article um matches the idea in English.

What does manga mean here? Is it the fruit mango?

No. In this context:

manga = sleeve (of a shirt, jumper, coat, etc.).

So em vez da manga means instead of (using) your sleeve.

The fruit mango is manga as well, but the meaning is clear here from context: we’re talking about what you sneeze into, not what you eat.

Why do we say da manga and not just de manga?

Da is the contraction of de + a:

de (of/from) + a (the, feminine singular) → da

Manga is feminine: a manga (the sleeve).
So:

em vez de + a manga → em vez da manga – instead of the sleeve.

The definite article a (the) is normal with body parts or clothing when they’re “possessed” by the subject, even if in English we’d say your sleeve:

Lava as mãos. – Wash your hands.
Puxa a manga. – Pull up your sleeve.

How would I say “your sleeve” explicitly instead of just da manga?

You can say:

em vez da tua manga – instead of your sleeve (informal tu)
em vez da sua manga – instead of your sleeve (formal você or 3rd person)

But in European Portuguese it’s completely natural and often more idiomatic to omit the possessive here and just say da manga, because it’s clear whose sleeve we’re talking about.

Could I say “Se espirrares muito” instead of “Se estiveres a espirrar muito”?

Yes, grammatically you can:

Se espirrares muito, usa um lenço de papel…

The meaning is still If you sneeze a lot, use a tissue…

The difference is nuance:

Se estiveres a espirrar muito – focuses on the action going on at that time, like if you’re sneezing a lot (right then).
Se espirrares muito – a bit more general, like if you sneeze a lot (whenever it happens).

Both can work in everyday language, but estar a espirrar strongly evokes the ongoing action.

Can I move the if-clause to the end: “Usa um lenço de papel em vez da manga se estiveres a espirrar muito”?

Yes. The word order is flexible:

Se estiveres a espirrar muito, usa um lenço de papel em vez da manga.
Usa um lenço de papel em vez da manga se estiveres a espirrar muito.

Both are correct and natural. Putting the se-clause at the beginning is slightly more common and gives it more emphasis, but there’s no change in meaning.

Is espirrar reflexive, like “espirrar-se”?

No. Espirrar (to sneeze) is not reflexive in Portuguese:

Eu espirro muito. – I sneeze a lot.
Quando espirras, tapa a boca. – When you sneeze, cover your mouth.

You don’t say espirrar-se in standard Portuguese for this meaning.

What would be a more formal version of this sentence?

To address someone formally (você) and sound a bit more polite, you could say:

Se estiver a espirrar muito, use um lenço de papel em vez da manga.

Changes:

estiveres → estiver (subjunctive for você)
usa → use (imperative form used for você)