Можно: permission and possibility

Можно can express permission:

  • Можно задать вопрос? — May I ask a question?
  • Можно открыть окно? — May I open the window?
  • Здесь можно сидеть. — One may sit here / sitting is allowed here.

It can also express possibility:

  • Это можно сделать быстро. — This can be done quickly.
  • Этот текст можно читать без словаря. — This text can be read without a dictionary.

The person affected can be marked with dative:

  • Мне можно уйти? — May I leave?
  • Детям можно играть здесь. — Children may play here.

Можно is invariant. It does not agree for gender or number.

Нужно and надо: necessity

Нужно and надо both express need or necessity, often with an infinitive:

  • Мне нужно уйти. — I need to leave.
  • Надо проверить ответ. — The answer needs to be checked.
  • Нам надо больше читать. — We need to read more.
  • Тебе нужно отдохнуть. — You need to rest.

Надо often feels more general, practical, or colloquial-neutral; нужно can feel slightly more explicit or formal in some contexts. They overlap heavily.

With a noun, нужен/нужна/нужно/нужны agrees with the thing needed:

  • Мне нужен словарь. — I need a dictionary.
  • Мне нужна помощь. — I need help.
  • Мне нужно время. — I need time.
  • Мне нужны примеры. — I need examples.

This is a major learner trap: нужно + infinitive is invariant; нужен with a noun agrees.

Нельзя: prohibition and impossibility

Нельзя can express prohibition:

  • Здесь нельзя курить. — Smoking is not allowed here.
  • Нельзя трогать документы. — The documents must not be touched.
  • Детям нельзя смотреть этот фильм. — Children are not allowed to watch this film.

It can also express impossibility:

  • Эту дверь нельзя открыть без ключа. — This door cannot be opened without a key.
  • Так нельзя переводить. — One cannot translate like that / this is not acceptable translation.

Context separates “not allowed” from “not possible.” Sometimes both are present.

Aspect matters:

  • Нельзя открывать дверь. — Opening the door is not allowed.
  • Нельзя открыть дверь без ключа. — It is impossible to open the door without a key.
  • Нельзя потерять документы. — The documents must not be lost.

Должен: obligation with agreement

Должен behaves like a short-form adjective and agrees with the subject:

  • Я должен уйти. — I must leave. (male speaker)
  • Я должна уйти. — I must leave. (female speaker)
  • Он должен подписать. — He must sign.
  • Она должна подписать. — She must sign.
  • Мы должны уйти. — We must leave.

Должен can mean must, have to, be supposed to, or owe depending on context.

  • Он должен деньги. — He owes money.
  • Доклад должен быть коротким. — The report should/must be short.

Because должен agrees, it belongs in the same mental area as short-form adjectives, not with invariant words like можно and нельзя.

The infinitive after modal predicates must be interpreted aspectually.

  • Нужно читать каждый день. — It is necessary to read every day.
  • Нужно прочитать текст. — It is necessary to read the text through.
  • Можно пользоваться словарём. — One may use a dictionary.
  • Можно взять словарь? — May I take the dictionary?
  • Нельзя опаздывать. — One must not be late / lateness is not allowed.
  • Нельзя опоздать. — We must not end up late.

This is the bridge to Article 140. Modality does not erase aspect.

Contrast sets

Можно: permission vs possibility:

  • Можно войти? — May I come in?
  • Это можно объяснить проще. — This can be explained more simply.

Нужно with infinitive vs нужен with noun:

  • Мне нужно уйти. — I need to leave.
  • Мне нужен билет. — I need a ticket.
  • Мне нужна карта. — I need a map.
  • Мне нужны документы. — I need documents.

Нельзя: rule vs impossibility:

  • Здесь нельзя шуметь. — Making noise is not allowed here.
  • Этот текст нельзя понять без контекста. — This text cannot be understood without context.

Надо vs должен:

  • Мне надо уйти. — I need to leave.
  • Я должен уйти. — I must leave / I am obligated to leave.

Common learner misreadings

The first mistake is to use должен for every English “must.” Russian often uses надо or нужно for practical necessity.

The second mistake is to forget agreement in должен and нужен. A female speaker says я должна, not я должен. A plural noun requires нужны.

The third mistake is to ignore the dative person: мне нужно, тебе можно, детям нельзя. English subjects do not map neatly onto Russian modal grammar.

Make separate modal frames:

  • можно + infinitive
  • кому можно + infinitive
  • нужно/надо + infinitive
  • кому нужно/надо + infinitive
  • кому нужен/нужна/нужно/нужны + noun
  • нельзя + infinitive
  • кому нельзя + infinitive
  • subject + должен/должна/должно/должны + infinitive

Practice one frame at a time. Mixing them too early causes durable errors.

Treat modal predicates as a system of permission, possibility, necessity, prohibition, obligation, and advisability. Do not present можно, нужно, надо, нельзя, and должен as a flat vocabulary list. Each item carries grammar, aspect behavior, and social force.

Start with the basic map:

  • Можно войти? — May I come in?
  • Здесь можно работать. — One may/can work here.
  • Нужно проверить данные. — The data need to be checked / it is necessary to check the data.
  • Надо уходить. — We/I/one need to leave.
  • Нельзя шуметь. — One must not make noise.
  • Это нельзя доказать. — This cannot be proven.
  • Я должен ответить. — I must answer / I am obliged to answer.

Distinguish impersonal modal predicates from agreeing short-form adjectives like должен.

Dative Controllers

Many modal sentences specify who is affected with dative:

  • Мне нужно уйти. — I need to leave.
  • Ему нельзя поднимать тяжёлое. — He must not lift heavy things.
  • Нам можно здесь сидеть? — May we sit here?
  • Тебе надо больше спать. — You need to sleep more.

The dative is not the subject in an English sense. It marks the person for whom the permission, prohibition, need, or state applies. Make that visible.

Можно: Permission Versus Possibility

Можно has two major readings:

  • Здесь можно курить? — Is smoking allowed here?
  • Этот текст можно прочитать за час. — This text can be read in an hour.

The first is permission. The second is possibility/feasibility. Context usually decides, but aspect helps: activity infinitives often appear in permission rules; perfective infinitives often appear in feasibility or task completion.

Нельзя: Prohibition Versus Impossibility

Нельзя also has two major readings:

  • Здесь нельзя парковаться. — Parking is not allowed here.
  • Эту ошибку нельзя исправить. — This mistake cannot be corrected.

Again, one is social/legal prohibition, the other impossibility. Learners who translate both as “must not” will misread many texts.

Надо Versus Нужно Versus Должен

Надо and нужно overlap heavily, but style and force vary by context. Нужно can sound more neutral or formal in many settings; надо is common and conversational but also widely used in serious speech. Должен is different because it agrees with the obligated person and can imply duty, obligation, or expectation:

  • Мне нужно закончить работу. — I need to finish the work.
  • Мне надо закончить работу. — I need to finish the work.
  • Я должен закончить работу. — I must / am obliged to finish the work.
  • Она должна ответить. — She must answer.
  • Они должны прийти вовремя. — They must arrive on time.

Do not pretend these are exact synonyms.

Practice Routine

For each modal sentence, label:

  1. Modal meaning: permission, possibility, necessity, prohibition, impossibility, duty.
  2. Controller: dative experiencer/affected person, nominative obligated person, or general rule.
  3. Infinitive aspect: activity/policy or bounded task/result.
  4. Register: sign, conversation, official instruction, personal obligation.

Drill:

  • “You may sit here.” → Здесь можно сидеть.
  • “This can be solved.” → Это можно решить.
  • “You must not enter.” → Входить нельзя. / Нельзя входить.
  • “This cannot be explained briefly.” → Это нельзя объяснить кратко.
  • “I must leave.” → Мне нужно уйти. / Я должен уйти. (different force)

Final rule

Russian modality is frame-based. Learn можно, нужно, надо, нельзя, and должен with their grammar, not as one-word translations of English modals.