Se la tabaccheria è chiusa, il tabaccaio lascia sempre un avviso sulla porta.

Yes, in this sentence se means if and introduces a condition:

• Se la tabaccheria è chiusa = If the tobacco shop is closed

This is one of the most common uses of se in Italian.
Be careful, though: se can also sometimes mean whether in other contexts, for example:

• Non so se viene = I don’t know whether he’s coming

So the meaning depends on the sentence.

Because they are two different nouns with different grammatical genders:

• la tabaccheria = the tobacco shop / tobacconist’s shop
  
  • feminine singular
• il tabaccaio = the tobacconist / shopkeeper
  
  • masculine singular

This is a useful pair to remember:

• tabaccheria = the place
• tabaccaio = the person

Italian often has this kind of distinction.

A tabaccheria is not just any shop. In Italy, it is a specific kind of licensed shop that traditionally sells tobacco products, but often also things like:

• stamps
• bus tickets
• lottery tickets
• stationery
• small convenience items

So although it literally relates to tobacco, culturally it refers to a very specific type of shop.

È chiusa means is closed. Here, chiusa is an adjective (or a past participle used like an adjective) describing the shop’s state.

• la tabaccheria è chiusa = the shop is closed

It has to agree with tabaccheria, which is feminine singular, so:

• masculine singular: chiuso
• feminine singular: chiusa
• masculine plural: chiusi
• feminine plural: chiuse

So you say:

• il negozio è chiuso
• la tabaccheria è chiusa

By contrast, chiude means closes / is closing, which describes an action, not a state.

Italian often uses the present tense for general truths, habits, or repeated situations, just as English can.

This sentence describes something that regularly happens:

• Se la tabaccheria è chiusa, il tabaccaio lascia sempre un avviso sulla porta.

That means something like:

• If the shop is closed, the tobacconist always leaves a notice on the door.

So the present tense here is not about this exact moment only; it expresses a habitual fact.

In Italian, after se referring to a real or likely condition, you normally do not use the future in the se clause.

So Italian says:

• Se la tabaccheria è chiusa...

not:

• Se la tabaccheria sarà chiusa... in this kind of normal conditional meaning

This is different from English, where learners may think of If the shop will be closed..., but that is usually unnatural in English too. The normal pattern is present + present, or present + future depending on meaning.

Examples:

• Se piove, resto a casa. = If it rains, I stay home / I’ll stay home.
• Se arriva, ti chiamo. = If he arrives, I’ll call you.

Sempre means always, and its position is very natural here:

• il tabaccaio lascia sempre un avviso

This is the usual placement for many adverbs in Italian: after the verb and before the direct object.

Other positions are sometimes possible for emphasis, but this is the most neutral and standard one.

Compare:

• Mangio sempre pane a colazione.
• Lei arriva sempre tardi.

So lascia sempre un avviso sounds completely normal.

Not usually. In this sentence, un avviso means something like:

• a notice
• a message
• a sign posted for people to read

So here it probably means a written notice on the door, such as opening hours or a message explaining the closure.

Although avviso can sometimes mean warning depending on context, here notice is the most natural understanding.

Sulla is the contraction of:

• su = on
• la = the

So:

• su + la = sulla

That gives:

• sulla porta = on the door

Italian very often combines prepositions with definite articles:

• su + il = sul
• su + la = sulla
• a + il = al
• di + il = del

You generally need the article here because Italian says on the door, not just on door.

Because Italian usually does not need them when the verb already shows the subject clearly.

• lascia already tells you it is he/she leaves
• the noun il tabaccaio is also explicitly stated

So adding lui would usually be unnecessary unless you want emphasis or contrast.

Italian often sounds more natural without subject pronouns:

• Parlo italiano. = I speak Italian.
• Arrivano domani. = They arrive tomorrow.

In this sentence, il tabaccaio lascia is perfectly clear without any pronoun.

Yes. Italian allows either order:

• Se la tabaccheria è chiusa, il tabaccaio lascia sempre un avviso sulla porta.
• Il tabaccaio lascia sempre un avviso sulla porta se la tabaccheria è chiusa.

Both are grammatical. The version with se first often sounds slightly more natural when you want to present the condition first.

Also notice the comma:

• when the se clause comes first, a comma is commonly used
• when it comes second, the comma is often omitted

In this sentence, it is best understood as an adjective-like form describing a condition:

• è chiusa = is closed

Grammatically, chiusa comes from the past participle of chiudere (to close), but here it is not being used to form a completed action like English has closed. Instead, it describes the resulting state.

So this is closer to:

• The shop is closed

than to:

• The shop has closed

That distinction is important in Italian.

You would need to change the article, noun, adjective, and possibly the rest of the sentence for agreement.

For example:

• Se le tabaccherie sono chiuse, i tabaccai lasciano sempre un avviso sulla porta.

Changes:

• la tabaccheria → le tabaccherie
• è chiusa → sono chiuse
• il tabaccaio → i tabaccai
• lascia → lasciano

This shows how much agreement matters in Italian.

It is neutral, standard Italian.

Nothing in it is especially formal or informal. The vocabulary and grammar are normal and natural for everyday written or spoken Italian.

That makes it a good model sentence for learning:

• a basic se clause
• agreement with chiusa
• common article + preposition contraction in sulla
• everyday word order