Lei si strucca sempre con calma dopo la festa.

It can be either in theory, because the verb form is the same for she and formal you.

But the capital L strongly suggests formal you. In Italian, people often write Lei with a capital letter when it means the polite form of you. If it were lei with a lowercase l, it would normally mean she.

So:

• Lei si strucca... = You remove your makeup... (formal)
• lei si strucca... = She removes her makeup...

In modern Italian, the capital letter for formal Lei is common but not absolutely required.

Because the verb is reflexive: struccarsi.

A reflexive verb shows that the subject does the action to themselves. Here, the person is removing their own makeup.

So:

• struccare = to remove makeup from someone / to take makeup off
• struccarsi = to remove one’s own makeup

The si is the reflexive pronoun used with third-person singular forms:

• mi strucco = I remove my makeup
• ti strucchi = you remove your makeup
• si strucca = she removes her makeup / you remove your makeup (formal)

The dictionary form is struccarsi.

The part strucca is the third-person singular present tense form of the verb. That matches both lei = she and Lei = formal you.

Present tense of struccarsi:

• mi strucco
• ti strucchi
• si strucca
• ci strucchiamo
• vi struccate
• si struccano

So Lei si strucca means she removes her makeup or you remove your makeup in the formal form.

Yes, they could leave Lei out.

Italian often drops subject pronouns because the verb ending already shows the person and number. So Si strucca sempre con calma dopo la festa is perfectly natural.

Including Lei can add:

• emphasis
• contrast
• politeness clarity
• disambiguation

So both are possible, but omitting the subject is very common in everyday Italian.

That is a very normal position for adverbs in Italian.

In this sentence, sempre modifies the whole action: si strucca sempre = always removes her makeup / always remove your makeup.

This word order is natural and common:

• Lei si strucca sempre...

Italian adverb placement is somewhat flexible, but this is one of the most standard patterns. You might also hear other arrangements for emphasis, but the sentence as given sounds completely normal.

Because con calma is a very common and idiomatic expression.

Literally, it means with calm, but in natural English it usually corresponds to:

• calmly
• slowly and calmly
• taking one’s time

Italian often prefers expressions like con calma where English might use a single adverb.

Compare:

• Parla con calma = He/She speaks calmly
• Fallo con calma = Do it calmly / Take your time with it

So si strucca sempre con calma sounds very natural and conversational.

Because dopo can directly take a noun phrase.

So the normal structure is:

• dopo la festa = after the party
• dopo cena = after dinner
• dopo il lavoro = after work

You do not need an extra preposition there.

The article la is used because festa is being treated as a specific event: the party.

It most naturally describes a habit.

The Italian present tense can express:

• actions happening now
• regular habits
• general truths

Here, sempre strongly pushes the sentence toward a habitual meaning. So it means that this is something she does regularly or characteristically after the party.

With informal you, you would use tu and the second-person singular form:

• Tu ti strucchi sempre con calma dopo la festa.

Usually, though, Italian would omit tu unless you want emphasis:

• Ti strucchi sempre con calma dopo la festa.

Compare:

• Lei si strucca... = formal you
• Tu ti strucchi... = informal you

No. Struccarsi is a very common and efficient verb, but Italian can also express the idea in other ways.

For example:

• togliersi il trucco = to take off one’s makeup
• rimuovere il trucco = to remove makeup

But struccarsi is especially natural when talking about removing your own makeup, so it is a very good choice here.

Because formal Lei uses third-person singular grammar.

Even though it means you, it behaves grammatically like she:

• Lei parla
• Lei si strucca
• Lei è arrivata / Lei è arrivato depending on the person addressed

So the reflexive pronoun is si, not ti.

That is one of the key things to remember about formal Lei in Italian: semantically it means you, but grammatically it works like a third-person form.