Aqua hodie calidior est quam heri.

Aqua is nominative singular, so it is the subject of the sentence.

You can tell because:

• est is singular, so it needs a singular subject.
• aqua is the noun in nominative form.
• The sentence is saying something about the water: it is warmer.

So aqua = water / the water, as the thing being described.

Latin has no definite or indefinite article, so there is no separate word for the or a/an.

That means aqua can mean:

• water
• the water
• sometimes even some water

The exact English translation depends on context. In this sentence, English often uses the water because that sounds natural.

Hodie is an adverb meaning today.

It does not change its form, and it tells you when the statement is true:

• Aqua hodie calidior est = The water is warmer today

So hodie modifies the whole idea of the sentence, especially the comparison.

Calidior means warmer.

It is the comparative form of calidus, which means warm or hot.

So the pattern is:

• calidus = warm
• calidior = warmer
• calidissimus = warmest

The ending -ior is a standard comparative ending in Latin.

This is a very common question.

Comparative adjectives in Latin do agree with the noun they describe, but they use third-declension endings, not first/second-declension endings like -a.

For comparatives:

• masculine nominative singular = -ior
• feminine nominative singular = -ior
• neuter nominative singular = -ius

So both masculine and feminine singular use calidior.

That is why:

• aqua calidior = warmer water / the water is warmer

not calidiora.

Quam means than and introduces the second part of the comparison.

So:

• calidior ... quam ... = warmer ... than ...

In this sentence:

• Aqua hodie calidior est quam heri
  = The water is warmer today than yesterday

It marks what the water is being compared with.

Heri is an adverb meaning yesterday.

Like hodie, it is indeclinable, which means its form does not change for case, number, or gender.

So quam heri means than yesterday.

More exactly, Latin is comparing today with yesterday in terms of the water’s warmth.

Latin often leaves out words that are easy to understand from context.

The full English sense is:

• The water is warmer today than it was yesterday.

But Latin does not need to repeat aqua or another verb such as erat. The listener can easily supply the missing idea.

So quam heri is a compact way of saying:

• than yesterday
• that is, than it was yesterday

Yes. Latin word order is more flexible than English word order because the endings do much of the grammatical work.

You could also say things like:

• Hodie aqua calidior est quam heri
• Aqua calidior hodie est quam heri

These all mean roughly the same thing.

The original order, Aqua hodie calidior est quam heri, is perfectly normal. Different orders can slightly shift emphasis, but not the basic meaning.

In normal prose, yes, est is the regular and expected verb.

Latin often uses forms of sum (to be) the same way English does in sentences like this:

• aqua calida est = the water is warm
• aqua calidior est = the water is warmer

Sometimes Latin can omit est, especially in poetry or very compressed style, but for a standard learner sentence, est should be there.

Not naturally in this sentence.

Latin sometimes compares things in two ways:

• comparative + quam
• comparative + ablative of comparison

For example, with nouns you can have a structure like Marco maior = greater than Marcus.

But here the comparison word is heri, which is an adverb meaning yesterday, not a noun in the ablative. So quam heri is the natural construction.

That is why quam is used here.