Learn Hardcore Russian: Новое грамматическое правило: модальные глаголы «мочь», «надо» и «должен», а также конструкция «можно + инфинитив». - New grammar rule: the modal verbs “мочь,” “надо,” and “должен,” as well as the construction “можно + infinitive.”

Modal verbs (модальные глаголы) express the speaker’s attitude toward the action—ability, necessity, obligation, permission.
• мочь – ability or possibility (“to be able to”)
• надо – necessity or “need to”
• должен – obligation or “must/ought to”

Do these modals conjugate like regular verbs?

• мочь conjugates normally:
– я могу, ты можешь, он/она может, мы можем, вы можете, они могут
• должен agrees in person, gender, and number (present tense):
– я должен/должна, ты должен/должна, он должен, она должна, мы/они должны
• надо is impersonal and invariable—it never changes form

How do I indicate who needs to do something with надо?

Because надо is impersonal, you add a dative noun or pronoun to mark the “subject”:
– Мне надо позвонить (I need to call)
– Тебе надо уйти (You need to leave)
The action itself is expressed by the infinitive that follows надо.

What’s the difference between using мочь and можно + infinitive for permission?

• мочь requires an explicit subject:
– Я могу войти? (Can I come in?)
– Он может помочь (He can help)
• можно + infinitive is impersonal and general:
– Здесь можно курить (You may smoke here)
– Здесь нельзя парковаться (You may not park here)

How do надо + infinitive and должен + infinitive differ in nuance?

• надо + infinitive expresses an external necessity or practical need:
– Мне надо купить хлеб (I need to buy bread)
• должен + infinitive conveys a stronger sense of duty or moral obligation:
– Я должен помочь брату (I must help my brother)

Which case do these modals take when you have an object?

Modals themselves don’t govern object cases—case is determined by the main verb in the infinitive.
• Я могу прочитать книгу (книга in accusative)
• Тебе надо ждать автобуса (автобус in genitive if you use ждать)

Does можно + infinitive ever change form to agree with gender or number?

No. можно is indeclinable and never agrees. It always stays можно + infinitive, regardless of subject or object.

Can I use мочь and можно interchangeably?

Not really:
• мочь focuses on a specific subject’s ability/permission (with a subject pronoun).
• можно states general permission/possibility impersonalized (no subject required).

Why is можно + infinitive called a construction and not a modal verb?

Because можно behaves like an impersonal predicate:
• It doesn’t conjugate.
• It always pairs with an infinitive to form meaning.
In Russian grammar, that fixed pattern is termed a “construction” rather than a standalone modal verb.