Si les estovalles són netes, la meva mare hi posa els plats abans de sopar.

Hi is a locative clitic pronoun. Here it means something like there, on it, or on them, referring back to a place already understood from the context.

So la meva mare hi posa els plats means: my mother puts the plates there / on it / on them

In this sentence, hi most naturally refers to the tablecloth / the table.

A key point: hi does not change for singular/plural or gender. Even though les estovalles is plural, the pronoun is still just hi.

In everyday Catalan, les estovalles is very commonly used as a plural noun for what English often treats as a singular item, the tablecloth.

Because it is grammatically plural, everything agrees with it in the plural:

• les estovalles
• són
• netes

So even if English uses a singular translation, Catalan treats this noun as plural in normal usage.

Because les estovalles is feminine plural.

That affects both the verb and the adjective:

• són = plural form of ser
• netes = feminine plural form of net

So the agreement is:

• singular feminine: és neta
• plural feminine: són netes

Since the subject is les estovalles, the sentence must use the plural forms.

Catalan normally uses the definite article before possessives in many cases, including with family members.

So:

• la meva mare = my mother
• el meu pare = my father

This is different from English, which usually says just my mother, without the.

So la meva mare is the normal standard Catalan form here.

Because sopar here is being used as an infinitive verb: to have dinner / to dine.

The pattern is: abans de + infinitive = before doing something

So:

• abans de sopar = before having dinner

If you said abans del sopar, then sopar would be a noun:

• before the dinner
• before dinner as an event or meal

Both structures can exist, but they are not the same grammar.

Because this sentence expresses a real or general condition.

In Catalan, with this kind of if sentence, it is normal to use:

• present tense after si
• and often present or future in the main clause

So:

• Si les estovalles són netes, la meva mare hi posa els plats.

This is similar to English sentences like:

• If the tablecloth is clean, my mother puts the plates there.

Catalan does not use a special future or conditional form after si in this kind of sentence.

Possibly, but the nuance changes a little.

Catalan often uses ser in places where an English speaker might expect a temporary-state verb. So són netes is perfectly natural for simply describing the tablecloths as clean.

Using estan netes would put more focus on their current state, especially as the result of having been cleaned.

Very roughly:

• són netes = they are clean
• estan netes = they are clean right now / they have ended up clean

In this sentence, són netes sounds normal and idiomatic.

Because in Catalan, most descriptive adjectives normally come after the noun.

So:

• estovalles netes = clean tablecloths
• plats blancs = white plates

This is the usual order in Catalan. Putting the adjective before the noun is possible in some cases, but it often sounds more literary, emphatic, or subjective.

With a conjugated verb, clitic pronouns like hi normally go before the verb:

• hi posa

That is why the sentence says:

• la meva mare hi posa els plats

Not:

• la meva mare posa-hi els plats

You do get pronouns after the verb in some other situations, such as:

• with an infinitive: posar-hi
• with a gerund: posant-hi
• with an affirmative command: posa-hi

But with the present tense here, hi goes before the verb.

Literally, posar means to put or to place.

But in a sentence about plates and dinner, English may translate it more naturally as:

• put the plates down
• set out the plates
• put the plates on the table

Catalan uses posar very broadly, so it often covers ideas that English expresses with several different verbs.