Learn Hardcore Russian: Зимой вода холоднее, чем летом. - In winter the water is colder than in summer.

So instead of saying something like Зимой вода есть холоднее, Russian just says Зимой вода холоднее. The adjective (or comparative) холоднее itself works as the predicate, so you understand “water is colder” without an explicit is.

In the past or future, Russian does use a form of быть:

Зимой вода была холоднее. – In winter the water was colder.
Зимой вода будет холоднее. – In winter the water will be colder.

What are зимой and летом grammatically? Why do they end in -ой and -ом?

Both зимой (from зима – winter) and летом (from лето – summer) are nouns in the instrumental singular form being used adverbially.

зима → зимой (instrumental singular)
лето → летом (instrumental singular)

In Russian, certain time nouns in the instrumental case are very commonly used to mean “in (the) X season/time”:

зимой – in (the) winter
летом – in (the) summer
осенью – in (the) autumn/fall
весной – in (the) spring

So the endings -ой and -ом show the instrumental case, and here that case has an adverbial meaning: when something happens.

Why don’t we say в зиму / в зиму воду холоднее or в зиму вода холоднее instead of зимой?

For seasons, Russian has a strong preference for using the instrumental without a preposition to mean “in (that season)”:

зимой – in (the) winter
летом – in (the) summer

Using в зиму is either:

poetic/archaic, or
used with a different meaning, like “into the winter (period)” rather than a simple “in winter.”

For everyday neutral speech, you say:

Зимой вода холоднее. – In winter water is colder.

You could also say зимой вода холодная or зимой здесь очень холодно, but you do not add в in front of зимой here.

Could we say в зимой or в летом?

No, в зимой and в летом are ungrammatical.

If you use the preposition в with a season, you normally use the prepositional case:
- в зиме, в лете – these forms exist but are rare and usually used in more abstract or poetic contexts, not for a simple “in winter / in summer” meaning.

For the basic time meaning “in winter / in summer”, Russian uses:

зимой, летом (instrumental, with no preposition).

So the natural, correct version is:

Зимой вода холоднее, чем летом.

Is холоднее a separate word or a form of холодный? How is this comparative formed?

холоднее is the comparative form of the adjective холодный (cold).

холодный – cold
холоднее – colder

This is the synthetic comparative, formed by changing the ending, not by adding a separate word. Many Russian adjectives form comparatives this way:

тёплый → теплее – warm → warmer
быстрый → быстрее – fast → faster
дешёвый → дешевле – cheap → cheaper

You can also make a periphrastic comparative using более:

более холодный – more cold / colder

So all of these are grammatical, but differ in style:

Зимой вода холоднее, чем летом. – most natural.
Зимой вода более холодная, чем летом. – also correct, a bit heavier/less neutral for such a simple adjective.

Why is there a comma before чем?

In Russian, comparisons with чем (than) are usually treated as a separate clause or clause‑like construction and normally take a comma:

Он выше, чем я. – He is taller than I am.
Это проще, чем кажется. – This is simpler than it seems.

So in your sentence:

Зимой вода холоднее, чем летом.

the comma is standard punctuation, marking the comparison introduced by чем. In simple colloquial sentences you may occasionally see it omitted in informal writing, but the correct written norm uses the comma.

Can we omit чем летом and just say Зимой вода холоднее.? Does it still sound complete?

Yes, you can say:

Зимой вода холоднее.

This is a complete and natural sentence. In context, it will be understood as:

“In winter, the water is colder (than at other times / than usual).”

Including чем летом makes the comparison explicit and specific:

Зимой вода холоднее, чем летом. – In winter the water is colder than in summer.

Without чем летом, the comparison is left general or implicit, but the sentence is still perfectly grammatical.

Could the word order be Вода зимой холоднее, чем летом? Is there any difference in meaning?

Yes, that word order is also correct:

Вода зимой холоднее, чем летом.

This version emphasizes вода (water) slightly more at the start. The original word order:

Зимой вода холоднее, чем летом.

puts зимой (in winter) in first position, so the time frame is emphasized.

Both mean the same thing. Word order in Russian is relatively flexible and is often used to shift emphasis or topic, not to change core meaning.

Why is вода in the nominative case?

вода is the subject of the sentence. In Russian, the subject is normally in the nominative case.

The structure is:

Зимой – adverbial modifier of time (when?)
вода – subject (what is colder?)
холоднее, чем летом – predicate (what is said about the subject?)

So вода is in the nominative singular: вода.

Could we add бывает and say Зимой вода бывает холоднее, чем летом? How does that change the meaning?

Yes, you can say:

Зимой вода бывает холоднее, чем летом.

The verb бывает adds the sense of “sometimes / tends to be / can be”:

Зимой вода холоднее, чем летом. – In winter, water is colder than in summer. (general fact)
Зимой вода бывает холоднее, чем летом. – In winter, the water can be / is sometimes colder than in summer. (not necessarily always, but often or at times)

So бывает makes the statement less absolute and more about typical or occasional occurrences.

Are зимой and зимою, летом and летою interchangeable?

Yes, in modern Russian:

зимой and зимою are both correct forms of the instrumental singular.
летом and летою are both correct forms of the instrumental singular.

However:

зимой, летом are the most common and neutral forms in everyday speech and writing.
зимою, летою often sound more poetic, old‑fashioned, or stylistically marked.

So in normal conversation or neutral prose you would typically say:

Зимой вода холоднее, чем летом.

Using зимою or летою is not wrong, but it may give the sentence a slightly literary or archaic flavor.