Ju pli varma estas la vetero, des pli da akvo mi trinkas.

This is the standard Esperanto pattern for the ... the ... comparisons.

So:

• Ju pli varma estas la vetero ... = The warmer the weather is ...
• des pli da akvo mi trinkas. = ... the more water I drink.

A simple way to think of it:

• ju introduces the changing condition
• des introduces the matching result

You will often see:

• Ju pli ..., des pli ... = The more ..., the more ...
• Ju malpli ..., des malpli ... = The less ..., the less ...
• Ju pli ..., des malpli ... = The more ..., the less ...

This is a very common Esperanto structure.

Because the sentence contains two linked comparisons.

• ju pli varma = the warmer
• des pli da akvo = the more water

In English, the two halves match each other:

• The warmer the weather is, the more water I drink.

Esperanto does the same thing very explicitly. The first pli measures the degree of warmth; the second pli measures the amount of water.

Because varma is an adjective describing la vetero.

• la vetero = the weather
• varma = warm

So pli varma estas la vetero literally means the weather is warmer.

If you used varme, that would be an adverb, meaning something like warmly or it is warm in an impersonal sense.

So these are different:

• Ju pli varma estas la vetero ... = The warmer the weather is ...
• Ju pli varme estas ... = The warmer it is ...

Both are possible ideas in Esperanto, but this sentence specifically talks about the weather as the thing that is warm, so varma is correct.

Because Esperanto word order is fairly flexible.

The speaker puts pli varma early because that is the key comparative idea after ju. So the sentence naturally starts with the comparison:

• Ju pli varma estas la vetero ...

This sounds smooth and idiomatic.

You could also understand a more straightforward order such as:

• Ju pli varma la vetero estas ...

and in some contexts even other arrangements may appear. But in comparative structures like this, Esperanto often places the comparative expression near ju and des.

The same thing happens in the second half:

• des pli da akvo mi trinkas

The important comparative part pli da akvo comes right after des.

Here da is used after a word of quantity.

So:

• pli da akvo = more water
• literally, something like more of water in quantity

In Esperanto, da is commonly used after words like:

• multe da = a lot of
• iom da = some
• sufiĉe da = enough
• pli da = more
• malpli da = less

Why not just pli akvo? Because Esperanto normally uses quantity word + da + noun when talking about an amount of something, especially an uncountable thing like water.

So:

• pli da akvo = correct
• pli akvo = not the normal standard form here

Because da is the preposition used for quantity or measure.

In this sentence, the speaker is talking about an amount of water, so Esperanto uses da:

• pli da akvo = more water

By contrast, de usually has other meanings, such as:

• possession
• origin
• source
• relationship

So pli de akvo would not be the normal way to say more water.

A useful rule of thumb is:

• use da after quantity expressions
• do not use de there

Because after da, the noun normally stays in its basic form.

So you say:

• pli da akvo not
• pli da akvon

In this sentence, pli da akvo is understood as the thing being drunk, even though akvo itself does not take -n.

Also, pli is an adverb, so you do not add -n to it either.

This is normal Esperanto usage with quantity expressions:

• Mi trinkas multe da akvo.
• Mi bezonas iom da helpo.
• Ŝi aĉetis pli da pano.

So the absence of -n on akvo here is not a mistake; it is exactly what you expect after da.

Yes. The comma is standard and helpful here because the sentence has two linked parts:

• Ju pli varma estas la vetero,
• des pli da akvo mi trinkas.

The comma separates the ju clause from the des clause.

In careful written Esperanto, you should definitely include it in this kind of structure.

Yes. That would also be a good Esperanto sentence, but it is slightly different in structure.

• Ju pli varma estas la vetero ... focuses on the weather
• Ju pli varme estas ... is more impersonal, like the warmer it is ...

In many situations, the meaning is very close. The original sentence is just a bit more explicit because it names la vetero directly.

So both are possible, but the given sentence is perfectly natural and grammatical.