В жару мне хочется чего-нибудь посвежее, чем кофе.

В жару is a common Russian expression meaning in hot weather / in the heat.

Here, в + accusative is used in some time-and-condition expressions:

• в дождь = in rainy weather
• в холод = in the cold
• в жару = in the heat

So this is not really about physical location. It is more like during a period of heat or when it’s hot.

Мне хочется is an impersonal construction. Literally, it is something like to me, it is desired or I feel like.

This is very common in Russian when talking about cravings, moods, or temporary wishes:

• Мне хочется спать. = I feel like sleeping / I’m sleepy.
• Мне хочется чая. = I feel like some tea.
• Мне хочется чего-нибудь сладкого. = I feel like something sweet.

Compared with я хочу, мне хочется usually sounds:

• softer
• more spontaneous
• more about a passing desire

So in this sentence, it sounds natural: in hot weather, the speaker feels like having something more refreshing.

This is one of the most common questions.

Что-нибудь is the nominative/accusative form.
Чего-нибудь is the genitive form.

After хочется, Russian very often uses the genitive when talking about an indefinite thing someone wants, especially food or drink:

• хочется воды
• хочется чаю
• хочется чего-нибудь вкусного

So чего-нибудь here means something like:

• something
• some kind of thing
• something to drink/eat

It has an indefinite, slightly partitive feel: not one specific item, just something.

Посвежее is the comparative form of свежий.

• свежий = fresh
• свежее = fresher / more refreshing
• посвежее = a bit fresher / something on the fresher side

The prefix по- with a comparative often softens it. It can make the comparison sound less blunt and more conversational.

So:

• свежее = fresher
• посвежее = somewhat fresher / a little fresher / something more refreshing

In this sentence, it sounds very natural and idiomatic.

Not only fresh in the literal sense.

With drinks and food, свежий can also mean:

• refreshing
• cool/light
• more suitable for hot weather

So чего-нибудь посвежее, чем кофе does not necessarily mean coffee is stale. It means the speaker wants something that feels more refreshing than coffee, especially because it is hot.

Because Russian often leaves out an obvious noun if the context makes it clear.

Here, the speaker clearly means something to drink. So Russian does not need to say:

• какого-нибудь напитка посвежее or
• чего-нибудь посвежее выпить

Just чего-нибудь посвежее is enough.

This kind of omission is very natural in Russian, especially with food and drink:

• Хочется чего-нибудь сладкого.
• Купи что-нибудь вкусненькое.
• Налей чего-нибудь покрепче.

Чем means than in comparisons.

After a comparative form like посвежее, Russian often uses:

• ... чем ... = ... than ...

So:

• посвежее, чем кофе = more refreshing than coffee

This is the standard comparison pattern:

• лучше, чем вчера = better than yesterday
• быстрее, чем поезд = faster than a train
• интереснее, чем фильм = more interesting than the movie

After чем in this kind of comparison, the compared word is normally just the thing being compared against.

With кофе, you do not see a case ending change because кофе is usually treated as an indeclinable noun.

A useful extra point:

• кофе is traditionally masculine in standard Russian
• in colloquial speech, neuter is sometimes heard, but masculine is still the norm learners are usually taught

In this sentence, though, there is no adjective or verb agreement showing its gender, so you just see кофе unchanged.

Because чем кофе is a comparative phrase attached to посвежее.

In standard Russian punctuation, a comparison introduced by чем is normally separated by a comma:

• лучше, чем раньше
• быстрее, чем я думал
• посвежее, чем кофе

So the comma here is expected.

It sets the scene first: as for hot weather / when it’s hot.

Russian word order is flexible, and the beginning of the sentence often gives the context or topic. So В жару at the front sounds very natural.

The sentence could be rearranged, but the emphasis would shift:

• В жару мне хочется... = in hot weather, I feel like...
• Мне в жару хочется... = for me, when it’s hot, I feel like...

The original order is smooth and idiomatic because it starts with the general situation, then moves to the speaker’s feeling.

Grammatically, Russian can form comparisons with более + adjective, but here it would sound much less natural.

Russian strongly prefers the simple comparative in everyday speech:

• посвежее rather than более свежего
• полегче rather than более лёгкого
• погорячее rather than более горячего

So чего-нибудь посвежее is the idiomatic choice. It sounds natural, spoken, and concise.