Tinc dues peres a la bossa; si en vols, te'n dono una.

Because dues is the feminine form of two, and peres is a feminine plural noun.

Catalan has:

• dos = two, masculine
• dues = two, feminine

Examples:

• dos llibres = two books
• dues peres = two pears

So dues peres agrees in gender with peres.

In Catalan, a can mean several things depending on context, including to, at, or in.

Here, a la bossa means in the bag. This is a normal Catalan way to express location with some containers or places.

So:

• Tinc dues peres a la bossa = I have two pears in the bag

English and Catalan do not always use the same preposition, so this is something you often just learn as a pattern.

Catalan often omits subject pronouns because the verb ending usually makes the subject clear.

In this sentence:

• tinc = I have
• vols = you want
• dono = I give

So Catalan does not need to say:

• Jo tinc...
• si tu en vols...
• jo te'n dono...

Those pronouns can be added for emphasis, but they are normally left out.

Here en is a partitive pronoun. It stands for some of them / any of them / of it, depending on context.

In this sentence, en refers back to the pears:

• si en vols = if you want some / if you want any / if you want one

A very literal version would be:

• if of-them you-want

English usually does not use a pronoun like this, but Catalan often does when talking about an unspecified quantity of something already mentioned.

Because en again refers back to the pears as the thing being taken from a group already mentioned.

• te'n dono una = I’ll give you one of them

This is very typical Catalan structure:

• en = of them / of it
• una = one

So the sentence does not just mean I give you one in a vague sense. It means I give you one of those pears.

Te'n is a combination of two weak pronouns:

• te = to you
• en = of them / of it

So:

• te'n dono una literally means something like I give you one of them

The apostrophe appears because these pronouns are combined in a fixed written form:

• te + en = te'n

You may also learn the equivalent form et for you, but in combinations like this, te is the form you normally see:

• te'n dono
• te'l dono
• te la dono

Because et dono una would only mean I give you one, without explicitly showing one of them.

The en is important because it refers back to the pears. It makes the connection clear:

• te'n dono una = I give you one of them

Without en, the sentence might still be understood from context, but it is less precise and less idiomatic here.

Because en tells you the thing comes from that group, but it does not tell you how many.

So:

• en = of them
• una = one

Together:

• te'n dono una = I give you one of them

Compare:

• te'n dono una = I give you one of them
• te'n dono dues = I give you two of them
• te'n dono = I give you some / I give you some of them

So una gives the quantity.

Yes. Vols is the 2nd person singular form of voler = to want.

Present tense of voler includes:

• vull = I want
• vols = you want
• vol = he/she/it wants

So:

• si en vols = if you want some / if you want any

More or less, yes. A literal breakdown is:

• si = if
• en = of them / some
• vols = you want

So the structure is much closer to:

• if you want some than to normal English word-for-word order.

English usually avoids this kind of partitive pronoun, but Catalan uses it naturally.

Because weak pronouns in Catalan normally go before the conjugated verb.

So:

• te'n dono = correct
• dono te'n = not correct

This is the normal pattern with a finite verb:

• el veig = I see him/it
• li dono un llibre = I give him/her a book
• te'n dono una = I give you one of them

Pronoun placement is a very important part of Catalan grammar.

People would probably understand you, but te'n dono una is better and more natural here.

Why?

• et dono una = I give you one
• te'n dono una = I give you one of them

Since the pears have already been mentioned, Catalan normally uses en to show that the one comes from that previously mentioned set.

So te'n dono una is the most idiomatic version.

Catalan often uses the present tense for an immediate offer or near-future action, just like English sometimes does.

So:

• te'n dono una can mean I’ll give you one
• It sounds natural and immediate, like making an offer on the spot

A future form is possible in other contexts, but the present works very well here.

The semicolon links two closely related clauses:

• Tinc dues peres a la bossa
• si en vols, te'n dono una

It shows a stronger break than a comma, but not as strong as a full stop. In English, you could also write this with a period or sometimes a comma, depending on style.

Its function here is mainly punctuation and clarity, not grammar.