Ti tengo da parte un pasticcino al limone per dopo cena.

Ti is the indirect object pronoun meaning to you or for you.

So:

• Ti tengo da parte... = I’m keeping/saving ... for you

Italian often uses this short pronoun instead of a full phrase like per te.
You could sometimes say Tengo da parte un pasticcino per te, but ti tengo da parte un pasticcino is very natural and common.

In Italian, unstressed object pronouns like mi, ti, gli, le, ci, vi usually go before a conjugated verb.

So:

• Ti tengo da parte...
• not normally Tengo ti da parte...

This is a basic word-order pattern in Italian. English says I’m saving you a pastry, but Italian puts the little pronoun before the verb.

Tenere da parte is an idiomatic expression meaning:

• to keep aside
• to save
• to set aside
• to reserve

So Ti tengo da parte un pasticcino means I’m setting a little pastry aside for you.

It is not just the literal meaning of tenere = to hold/keep plus da parte separately; together they make a very common expression.

Italian often uses the present tense to talk about something intended or arranged for the near future.

So:

• Ti tengo da parte un pasticcino can mean I’m saving a pastry for you / I’ll save a pastry for you

Using the future is possible in some contexts, but the present sounds very natural for a current intention or decision.

Da parte literally suggests the idea of to one side / aside.

In many contexts it means:

• aside
• separately
• put away

Examples:

• Metto i soldi da parte = I save money
• Tienilo da parte = Keep it aside

In your sentence, it gives the idea that the pastry is being kept separate so someone can have it later.

Italian usually uses an article where English may or may not.

• un pasticcino = a little pastry / a pastry

Leaving out the article would sound unnatural here. Italian normally needs that determiner with a singular countable noun.

Also, pasticcino is a small pastry / little cake / petit four-type sweet, from pasticcio/pasticceria vocabulary.

Al limone here means lemon-flavored or with lemon.

It comes from:

• a + il = al

This structure is very common for flavors:

• gelato al cioccolato = chocolate ice cream
• torta alla vaniglia = vanilla cake
• caramelle alla menta = mint candies

Di limone can exist in some contexts, but al limone is the usual, natural choice for flavor in this kind of phrase.

Literally, it is something like for after dinner.

• per = for
• dopo cena = after dinner

So the idea is: the pastry is being saved to eat later, specifically after dinner.

Italian often says dopo cena without an article, just like fixed time expressions such as:

• dopo pranzo = after lunch
• dopo scuola = after school

Because dopo cena is a common set expression meaning after dinner in a general sense.

Compare:

• dopo cena = after dinner, after the evening meal in general
• dopo la cena = after the dinner, after a specific dinner

In your sentence, dopo cena is more natural because it refers to the usual mealtime, not one specially identified dinner event.

Yes, to some extent. Italian word order is flexible, although some versions sound more natural than others.

For example:

• Ti tengo da parte un pasticcino al limone per dopo cena.
• Per dopo cena, ti tengo da parte un pasticcino al limone.

Both work.

But the original order is very natural because it flows like this:

1. for you → ti
2. I’m keeping aside → tengo da parte
3. what → un pasticcino al limone
4. for when → per dopo cena

Yes. Both are possible, but there is a slight nuance.

• Ti metto da parte un pasticcino = I’ll put a pastry aside for you
• Ti tengo da parte un pasticcino = I’m keeping a pastry aside for you

Mettere da parte emphasizes the action of setting it aside.
Tenere da parte emphasizes keeping it reserved.

In everyday speech, both are common and natural.

No. You can change it depending on who the pastry is being saved for:

• Mi tengo da parte... = I keep ... for myself
• Ti tengo da parte... = I keep ... for you
• Gli tengo da parte... = I keep ... for him
• Le tengo da parte... = I keep ... for her
• Ci tengo da parte... = I keep ... for us
• Vi tengo da parte... = I keep ... for you all
• Gli tengo da parte... = I keep ... for them

So ti is just the to you / for you version.

It is informal, because of ti.

If you were speaking formally to one person, you would usually say:

• Le tengo da parte un pasticcino al limone per dopo cena.

Here Le means for you in the formal sense.

So the sentence you gave sounds warm, everyday, and conversational.