Learn Hardcore Russian: Без паспорта нельзя пересечь границу. - Without a passport it is impossible to cross the border.

Grammatically, нельзя is an impersonal predicative word (often treated like a special kind of adverb). It doesn’t have a subject and is usually used with an infinitive:

Здесь нельзя курить. – You must not / cannot smoke here.
Тебе туда нельзя. – You are not allowed to go there / You can’t go there.

What is the grammatical role of нельзя in this sentence?

Нельзя is the main “verb-like” element of an impersonal construction:

Без паспорта нельзя пересечь границу.
Literally: Without passport, it-is-not-allowed / impossible to-cross border.

There is no subject (no “I/you/they”). The structure is:

нельзя + infinitive → “it is not allowed / it is impossible to …”

So нельзя functions like “it’s impossible / it’s not allowed” and the infinitive пересечь tells us what is impossible/not allowed.

Why is the verb in the infinitive, пересечь, and not a finite form like пересечёшь or пересечёте?

Because нельзя in this meaning normally takes an infinitive, not a personal (conjugated) verb:

нельзя курить – you must not smoke / smoking is not allowed
нельзя опаздывать – you must not be late
нельзя пересечь границу – you can’t / mustn’t cross the border

If you used a finite form, you would need an explicit subject:

Ты не пересечёшь границу без паспорта. – You will not cross the border without a passport.

Using нельзя + infinitive keeps it impersonal and general, like a rule that applies to anyone.

Why is it пересечь, not пересекать? What is the aspect difference here?

Пересечь is perfective, пересекать is imperfective.

пересечь границу – to cross the border (as a single, complete action)
пересекать границу – to be crossing / to cross (habitually, repeatedly, or as a process)

With нельзя, both are grammatically possible, but the nuance changes:

Без паспорта нельзя пересечь границу.
Focus: you cannot complete a single act of crossing the border without a passport. (This sounds like a concrete action: you’re trying to cross now / in a specific situation.)
Без паспорта нельзя пересекать границу.
Focus: one must not cross the border (ever, in general) without a passport. (Sounds more like a general rule, a repeated or habitual action.)

In practice, the perfective пересечь is very natural here, because border-crossing is typically seen as a clearly one-time completed act.

Why is границУ and not граница or границы?

Граница is a feminine noun:

nominative singular: граница – (the) border
accusative singular (for animate/inanimate feminine ending in -a): границу

The verb пересечь takes a direct object in the accusative case:

пересечь что? – границу (to cross what? the border)

So:

пересечь границу – to cross the border
перейти улицу – to cross the street
переплыть реку – to swim across the river

The -у ending here just marks the accusative singular of a feminine noun ending in -а.

Can I change the word order and say Нельзя пересечь границу без паспорта? Does it change the meaning?

Yes, that word order is also correct:

Без паспорта нельзя пересечь границу.
Нельзя пересечь границу без паспорта.

The basic meaning is the same. The difference is emphasis:

Без паспорта нельзя пересечь границу.
Starts with без паспорта, highlighting the condition: “Without a passport, you can’t cross the border.”
Нельзя пересечь границу без паспорта.
Starts with нельзя, highlighting the prohibition/impossibility: “You can’t cross the border without a passport.”

Both are natural; context and speaker focus decide which feels better.

Could I also say Без паспорта нельзя пересекать границу? How does that sound?

Yes, Без паспорта нельзя пересекать границу is grammatically correct. The nuance:

пересечь (perfective) – one specific act of crossing:
- Без паспорта нельзя пересечь границу.
  You can’t cross this border (now/in a specific situation) without a passport.
пересекать (imperfective) – repeated or general actions:
- Без паспорта нельзя пересекать границу.
  You must not (ever, as a rule) cross the border without a passport.

In many real situations, they will be understood almost the same, but the imperfective stresses the general rule, the perfective often feels more like a concrete attempt or event.

How is this different from saying Ты не можешь пересечь границу без паспорта?

Ты не можешь пересечь границу без паспорта literally addresses “you”:

Ты – you (singular, informal)
не можешь – cannot / are not able (2nd person singular)

So:

Ты не можешь пересечь границу без паспорта. – You cannot cross the border without a passport.

Differences:

Personal vs impersonal
- Нельзя пересечь границу без паспорта. – Impersonal, general rule; no subject.
- Ты не можешь... – Explicitly about you.
Style
- нельзя + infinitive sounds like a rule, regulation, or sign.
- не можешь is just describing your ability/possibility (which may come from the rule, but grammatically it’s about you).

Both can describe the same real-world situation, but the нельзя version sounds more like “This is the rule”, while не можешь sounds like “You won’t be able to do it”.

What’s the difference between пересечь границу and перейти границу?

Both can mean “to cross the border”, but there are nuances:

пересечь границу
- Very common collocation.
- Slightly more formal/neutral, often used in official or written contexts.
- Often used about borders, lines, distances:
  - пересечь границу, пересечь линию, пересечь страну.
перейти границу
- Literally: “to go across the border (mostly on foot)”.
- More concrete, sometimes more physical/colloquial.
- Very common with smaller obstacles:
  - перейти улицу – to cross the street
  - перейти дорогу – to cross the road

In the context of passports and regulations, пересечь границу is the standard phrase.

Is there some hidden subject like “one” or “you” in нельзя пересечь границу?

Grammatically, no subject is present. It’s an impersonal sentence.

However, in meaning, it applies to a generic person – like English “you” in “You can’t cross the border without a passport”, or “one” in “One cannot cross the border...”.

Russian often uses impersonal constructions to express general rules:

Здесь нельзя парковаться. – You can’t / mustn’t park here.
Тебе туда нельзя. – You’re not allowed to go there.

So while there is no grammatical subject, speakers mentally understand: “no person can do it”.

Does нельзя always mean “forbidden”, or can it also mean “physically impossible”?

Нельзя can express both:

Prohibition (rule, ban):
- Здесь нельзя курить. – Smoking is not allowed here.
- Нельзя опаздывать на экзамен. – You must not be late for the exam.
Impossibility (it cannot be done):
- Отсюда тебя не видно, нельзя рассмотреть лицо. – From here I can’t see you; it’s impossible to make out the face.
- Без ключа нельзя открыть дверь. – Without the key, it is impossible to open the door.

In Без паспорта нельзя пересечь границу, it naturally combines both:
According to rules, it’s forbidden, and as a result, it’s effectively impossible.

Why is there no word for “a/the” (article) before паспорт and границу?

Russian simply does not have articles like English a/an/the.

Words like паспорт and граница can correspond to:

a passport / the passport
a border / the border

The exact meaning is taken from context:

Без паспорта нельзя пересечь границу.
- In real-life context, this is understood as:
  You can’t cross the (state) border without a passport.

If you need to be more specific, Russian uses other means (word order, context, demonstratives like этот, тот, possessives, etc.), but not articles.