Com més àcida és la llimona, menys vinagre hi poso.

It is a very common Catalan pattern meaning the more..., the less....

So:

• Com més àcida és la llimona... = The more acidic the lemon is...
• ...menys vinagre hi poso. = ...the less vinegar I add to it.

This structure expresses a proportional relationship, just like English:

• The more you study, the better you do.
• Com més estudies, millor et va.

Yes, com often means how or like/as, but here it is part of a fixed comparative structure:

• com més... = the more...
• com menys... = the less...

So in this sentence, com is not asking a question. It is introducing the first half of the comparison.

A useful pattern to remember is:

• Com més X, més Y = The more X, the more Y
• Com més X, menys Y = The more X, the less Y

Because àcida agrees with la llimona, which is feminine singular.

Agreement in Catalan works like this:

• àcid = masculine singular
• àcida = feminine singular
• àcids = masculine plural
• àcides = feminine plural

Since llimona is feminine singular, the adjective must also be feminine singular:

• la llimona àcida
• Com més àcida és la llimona...

That word order is natural in this kind of comparative structure.

Catalan often uses:

• Com més + adjective + verb + subject

So:

• Com més àcida és la llimona...

is a normal and idiomatic way to say it.

For an English speaker, it may feel unusual because English often keeps the subject earlier. But in Catalan, this order sounds very natural here.

A version like Com més àcida la llimona és... would sound awkward or very marked.

Hi is a clitic pronoun. Here it refers to the place or mixture where the vinegar is being added.

So hi poso vinagre means something like:

• I put vinegar in it
• I add vinegar to it

In a cooking context, Catalan often uses hi when the destination is understood from context.

So menys vinagre hi poso is literally close to I put less vinegar in it, even if English would often just say I add less vinegar.

Sometimes in casual speech people may omit it if the context is very obvious, but in standard and careful Catalan, hi is very natural and often expected here.

Compare:

• Poso vinagre. = I put/add vinegar.
  This says what you add, but not clearly where.
• Hi poso vinagre. = I put/add vinegar to it.
  This makes the construction feel complete.

So in this sentence, hi is a good and idiomatic choice.

Because vinagre is being used as an uncountable substance, like water, salt, or sugar.

After menys, Catalan normally uses the noun without an article:

• menys vinagre = less vinegar
• menys sal = less salt
• menys aigua = less water

Using el vinagre here would usually not fit this structure.

It can be understood generically, especially in a cooking or recipe context.

Catalan often uses the definite article for general statements:

• La llimona can mean the lemon in a generic sense, or lemon as an ingredient/type.

So this sentence does not have to mean one specific lemon on one specific occasion. It can mean something like:

• When the lemon is more acidic, I add less vinegar
• The more acidic the lemon, the less vinegar I add

In other words, it can describe a general rule or habit.

The present tense is used because the sentence expresses a general rule, habit, or repeated practice.

• és = is
• poso = I put / I add

In English, we also often use the present for this kind of statement:

• The more acidic the lemon is, the less vinegar I add.

So the present tense here does not only refer to right now. It describes what normally happens.

àcida is pronounced roughly like AH-si-da.

A few helpful points:

• the à shows that the stress falls on that syllable: À-ci-da
• the c before i is pronounced like s
• so it begins AH-si-...

The accent mark helps show the stressed syllable and also distinguishes the vowel quality in standard spelling. For a learner, the most important thing is to stress the first syllable:

• Àcida, not a-CI-da or a-ci-DA