Learn Hardcore Russian: Зимой нельзя ездить на велосипеде по шоссе. - In winter one must not ride a bicycle on the highway.

В зиму exists but has a different, more literal meaning: into the winter (e.g. describing a transition from autumn into winter), and it’s not used for simple “in winter” in everyday speech.

So Зимой нельзя… literally = In winter (it is) not allowed…

What exactly does нельзя mean here? Is it “not allowed” or “impossible”?

Нельзя is an impersonal word that can mean:

Not allowed / forbidden – a rule, law, social norm:
- Зимой нельзя ездить на велосипеде по шоссе.
  → It’s not allowed / You mustn’t ride a bicycle on the highway in winter.
(Practically) impossible / can’t – because of physical limits:
- Здесь нельзя пройти. → You can’t get through here.

In your sentence, the default reading is prohibition: it is not allowed (for safety, rules, etc.). But in context it can also suggest it’s effectively impossible/dangerous.

There is no direct equivalent of English “must not / cannot / may not” — нельзя covers all of these depending on context.

Why is there no subject like “you” or “people” in the Russian sentence?

The construction with нельзя is impersonal: it has no grammatical subject.

Зимой нельзя ездить…
Literally: In winter it-is-not-allowed to-ride…

Russian uses such impersonal structures to express general rules, similar to English “You can’t…” / “One must not…” / “It is forbidden to…”.

The “you/people/one” meaning is implicit in нельзя. You don’t add a pronoun unless you want to stress who specifically is not allowed:

Тебе зимой нельзя ездить на велосипеде по шоссе.
You (in particular) must not ride a bike on the highway in winter.

Why do we use ездить and not ехать here?

Russian has special verbs of motion. For “to go by vehicle”:

ехать – one-direction, a single trip, “to be going (now)”
ездить – multi-direction / repeated / habitual, “to go (by vehicle) in general or regularly”

In a general rule like this, you want the habitual / repeated idea:

Зимой нельзя ездить на велосипеде по шоссе.
→ In winter you mustn’t ride bicycles on the highway (as a general rule).

If you said:

Зимой нельзя ехать на велосипеде по шоссе.

it would sound more like a specific situation: Right now / on this trip in winter you mustn’t go by bike on the highway. Both can be correct, but ездить is more natural for a standing rule.

Why is it на велосипеде and not на велосипед or just велосипедом?

На велосипеде is на + prepositional case and is the normal way to say “by bicycle / on a bicycle”:

ездить на велосипеде – to ride/go by bicycle
ехать на автобусе – to go by bus
лететь на самолёте – to fly by plane

Compare:

на велосипед (accusative) – onto the bicycle (direction, movement onto):
- Сесть на велосипед. – To sit/get onto a bike.
велосипедом (instrumental) – with/by means of a bicycle is grammatically possible but sounds bookish and unusual in modern speech for transport. People virtually always say на велосипеде for “by bike”.

So: ездить на велосипеде is the idiomatic, everyday form.

What does по шоссе literally mean, and which case is шоссе in?

По шоссе literally means “along the highway / on the highway as a route”.

по with motion verbs often means along / over / around a surface or route:
- идти по улице – walk along the street
- ехать по дороге – drive along the road
- гулять по парку – walk around the park

With по, the noun is usually in the dative. But шоссе is an indeclinable neuter noun: its form is the same in all cases (nom., gen., dat., etc.). So шоссе here is grammatically dative singular, even though it looks like nominative.

Hence: по шоссе = along the highway.

What’s the difference between по шоссе and на шоссе?

They can overlap in meaning but focus on different things:

по шоссе – emphasizes movement along the highway as a route:
- ездить по шоссе – to ride/drive along the highway.
на шоссе – emphasizes location on the highway:
- Машина стоит на шоссе. – The car is on the highway (located there).

In your sentence, the rule is about moving along the highway, so по шоссе is more natural: you mustn’t ride your bike along / on the highway as a route in winter.

Why doesn’t шоссе change its form? Shouldn’t there be something like шоссю?

Шоссе is an indeclinable noun; its form never changes:

Nom.: шоссе – the highway
Gen.: нет шоссе – there is no highway
Dat.: к шоссе / по шоссе
Acc.: видеть шоссе
Prep.: о шоссе

Russian has a number of such indeclinable nouns (especially foreign ones), e.g.:

кафе, метро, кино, пальто, такси

So по шоссе is correct; a form like шоссю does not exist.

Can the word order be different, like Нельзя зимой ездить на велосипеде по шоссе?

Yes. Russian word order is flexible. All of these are grammatically correct, with slightly different emphasis:

Зимой нельзя ездить на велосипеде по шоссе.
Neutral; sets the time frame first: In winter, it’s not allowed…
Нельзя зимой ездить на велосипеде по шоссе.
Emphasizes the prohibition нельзя first: It’s not allowed in winter to ride…
Ездить зимой на велосипеде по шоссе нельзя.
Strong focus on нельзя at the end; often sounds more categorical: Riding a bike on the highway in winter – that is not allowed.

All mean essentially the same in normal conversation.

Does this sentence talk about “you” specifically, or people in general?

It talks about people in general. Impersonal нельзя + infinitive usually means:

You mustn’t… (generic “you”)
One must not…
People aren’t allowed to…

So:

Зимой нельзя ездить на велосипеде по шоссе.
→ In winter you mustn’t / people mustn’t ride a bicycle on the highway.

To refer to a specific person or group, you add a pronoun or noun in dative:

Детям зимой нельзя ездить на велосипеде по шоссе.
Children mustn’t ride a bicycle on the highway in winter.

What’s the difference between ездить на велосипеде and кататься на велосипеде?

Both can translate as “to ride a bicycle”, but the nuance differs:

ездить на велосипеде
- Neutral, often practical: using a bike as transport or ability:
  - Я каждый день езжу на велосипеде на работу.
    I ride a bike to work every day.
  - Он умеет ездить на велосипеде.
    He can ride a bike.
кататься на велосипеде
- More about riding for fun / recreation, going around without a clear practical destination:
  - Мы любим кататься на велосипеде в парке.
    We like cycling in the park (for fun).

In your sentence, ездить на велосипеде по шоссе is natural because riding on a highway is seen as transport / purposeful travel, not just playful riding.

Could we use можно instead of нельзя, and how would the meaning change?

Yes, grammatically you can:

Зимой можно ездить на велосипеде по шоссе.
→ In winter it is allowed / you can ride a bicycle on the highway.

So:

можно = permitted / possible
нельзя = not permitted / not possible

They are basically opposites in this impersonal construction:

Зимой можно… – In winter it’s OK.
Зимой нельзя… – In winter it’s not OK / forbidden.