No em menjaré la pera fins que la rentis.

Here, menjar-se is a very common pronominal version of menjar.

So No em menjaré la pera still means I won’t eat the pear, but the em adds a more everyday, colloquial nuance, often something like eat up, go ahead and eat, or eat for myself.

Compare:

• No menjaré la pera = neutral, straightforward
• No em menjaré la pera = also natural, but a bit more colloquial or emphatic

In English, we usually do not translate that em directly.

Yes. That sentence is completely grammatical.

The difference is mainly nuance:

• No menjaré la pera... = more neutral
• No em menjaré la pera... = more colloquial, and can sound a bit more deliberate, like I’m not going to eat that pear...

So the version with em is not required, but it is very common in real Catalan.

Because in standard Catalan, the unstressed 1st-person singular pronoun before a verb is normally em.

Examples:

• em menjo
• em dic
• em llevo

Before a vowel or mute h, it usually becomes m':

• m'agrada
• m'he llevat

You may sometimes see me in some dialects or in certain enclitic combinations, but em menjaré is the standard form here.

Because the speaker is talking about a future action: the eating has not happened yet.

So menjaré means I will eat. With the negation:

• No em menjaré la pera... = I won’t eat the pear...

English also uses the future here: I won’t eat the pear until...

Catalan could sometimes use the present for near-future meaning in other contexts, but menjaré is perfectly normal and very natural here.

Because la pera means the pear.

Catalan uses the definite article in the normal way here: it refers to a specific pear that the speakers have in mind.

• la pera = the pear
• una pera = a pear

So this sentence is talking about a particular pear, not just pears in general.

Fins que means until when it is followed by a clause with a conjugated verb.

Here:

• fins que la rentis = until you wash it

You use fins que because rentis is a full verb form.

By contrast, fins a is used before nouns or time/place expressions, for example:

• fins a demà = until tomorrow
• fins a casa = as far as home / up to the house

So in this sentence, fins que is the correct choice.

Because after fins que, Catalan normally uses the subjunctive when the action has not happened yet and is still pending in the future.

In this sentence, the washing has not happened yet, so Catalan uses:

• rentis = present subjunctive

English does not show this difference clearly; it just says until you wash it.

So:

• No em menjaré la pera fins que la rentis
• literally: I won’t eat the pear until you wash it

The subjunctive is there because the washing is still only expected, not completed.

Rentis is:

• the present subjunctive
• 2nd person singular
• of the verb rentar = to wash

Its subjunctive pattern is:

• jo renti
• tu rentis
• ell/ella renti
• nosaltres rentem
• vosaltres renteu
• ells/elles rentin

So here rentis means you wash in a subjunctive environment.

That la is not the article the. It is a direct object pronoun meaning it.

It refers back to la pera.

So:

• la pera = the pear
• la rentis = you wash it

This is very common in Catalan. Once the noun is already known, Catalan often uses an object pronoun instead of repeating the noun.

Because unstressed object and reflexive pronouns in Catalan normally go before a conjugated verb.

So:

• em menjaré
• la rentis

This is the usual position.

You will often see them attached to the end only with infinitives, gerunds, and affirmative commands:

• menjar-me
• rentar-la
• renta-la

But with finite verb forms like menjaré and rentis, they normally come before the verb.

It is not written because Catalan often leaves subject pronouns out when the verb ending already shows the person.

Here, rentis already tells us the subject is tu.

So:

• fins que la rentis = until you wash it

If you wanted extra emphasis, you could say:

• fins que tu la rentis

But normally Catalan just omits tu.