Long before Hollywood pop-culture began flooding from “Pindostan” into the ex-USSR, the distinctively Latin American genre known as telenovelas (теленовеллы or телесериал) had attained cult status in Russia (particularly, but not exclusively, among women).

So in this post, let’s picture a scene from an imaginary теленовелла. Our beautiful (but naive) heroine María Simplemente has just been dumped by her boyfriend, Carlos Ricostambien, and has fled to the home of her best friend Dolores Fuertes de Barriga (known to her friends as Lola). And pay special attention to the
modal expressions marked in red:

МАРИЯ: О, Карлос, любимый мой! Как я скучаю по моему дорогому Карлосу! Почему он меня бросил? Горе мне!
MARIA: Oh, Carlos, my beloved! How I miss my darling Carlos! Why did he dump me? Woe is me!

ЛОЛА: Мария, пожалуй, тебе стоит позвонить Карлосу по телефону.
LOLA: Maria, perhaps it might make sense for you to call Carlos on the phone.

МАРИЯ: Не хочу! Не о чём разговорить с этим гадом!
I don’t want to! I’ve got nothing to talk about with that scumbag!

ЛОЛА: Милая, если тебе не угодно, тогда ты можешь не звонить Карлосу.
LOLA: Honey, if you don’t feel like it, then you don’t have to call Carlos.

МАРИЯ: Ну, конечно, мы с ним заняли у моего отца десять тысяч долларов, и Карлос ещё не выплатил долг…
Well, of course, he and I did borrow $10,000 from my father, and Carlos still hasn’t repayed the debt…

ЛОЛА: Значит, тебе надо позвонить Карлосу, ради твоего папы.
LOLA: So then, you have to call Carlos, for your dad’s sake.

МАРИЯ: Да, знаю. Только, я немного боюсь сообщать Карлосу, о ребёнке…
Yes, I know. But I’m just a little afraid to tell Carlos about the baby…

ЛОЛА: Как? Какой ребёнок? Мария, ты залетела?
LOLA: Huh? What baby? Maria, did you get knocked up?

МАРИЯ: Да, я беременная.
MARIA: Yes, I’m pregnant.

ЛОЛА: То ты обязана поговорить об этом с Карлосом! Тебе придётся позвонить ему!
LOLA: In that case you have a moral obligation to talk about this with Carlos! You’ve got no choice but to call him!

МАРИЯ: Довогорились, я позвоню моему милому Карлосу! Кажется, нам с ним суждено быть вместе, и я его очень люблю! Правда, он иногда угрожает мне ножом — время от времени. А только кодга он в трезвом состоянии…
MARIA: It’s settled, then. I’ll call my darling Carlos! I really think he and I were destined to be together, and I love him so much! True, sometimes he threatens me with a knife — from time to time. But only when he’s sober…

[Лола, в тревоге, выхватывает телефон у Марии]
[Lola, alarmed, snatches the phone from Maria]

ЛОЛА: Мария, милочка, лапушка, тебе не стоит звонить Карлосу. Тебе не надо звонить Карлосу. Тебе нельзя звонить Карлосу!
LOLA: Maria, honey, sweetie, it’s not worth it for you to call Carlos. You shouldn’t call Carlos. You’re not allowed to call Carlos!

МАРИЯ: Мне приходится позвонить Карлосу!
MARIA: I absolutely must call Carlos!

[Лола бросает мобильник в аквариум]
[Lola throws the cellphone into the fishtank]

ЛОЛА: Вот, теперь ты не можешь звонить Карлосу! В конце концов, я тебе запрещаю звонить Карлосу!
LOLA: Now you’re unable to call Carlos! I forbid you to call Carlos, and that settles it — end of story!

[тихо звонит телефон. Лола вытаскает его из воды.]
[the phone rings quietly. Lola hauls it out of the water.]

ЛОЛА: Аллё? Мария не может к телефону, она занята. А кто её просит? Карлос? Слушай, подлый урод ты, тебе не придётся поговорить больше с Марией!
LOLA: Hello? Maria can’t come to the phone, she’s busy. Who is this? Carlos? Listen, you lowlife creep, you don’t get to talk to Maria anymore!

* * * * * * * *

And now for a bit of analysis…

What’ll happen with Maria, Carlos, and the baby? Who cares — they’re just characters from a non-existent soap opera! But let’s talk a little more about “can’t”, “shouldn’t”, and other negated modal expressions in Russian. First, let’s recall that in non-negated sentences, the idea of “should, must, have to” can be rendered by different expressions having various shades of strength.

Кому-нибудь стоит делать что-нибудь.
Someone probably ought to do something.

(Or, “it wouldn’t be a bad idea”; or “it’d make sense”; “it would be worthwhile”, etc.) In other words, стоит can be translated “should,” but it’s a rather weak “should.”

Кому-нибудь следует делать что-нибудь.
Кому-нибудь надо делать что-нибудь.
Кто-то должен делать что-нибудь.
Someone should / has to / must do something.

These three are all intermediate in strength, and fairly similar in their shades of meaning — although следует can have the “it would probably make sense” implication of стоит, while должен can imply that the necessity to do something comes from some internal moral obligation. But to make that “ethical obligation” sense totally unambiguous, you can say, instead:

Кто-то обязан делать что-нибудь.
Someone is obliged (by morality/ethics/law) to do something.

Обязан is the masculine form, incidentally; like должен, it changes endings when the subject is feminine or plural: обязана, обязаны. The word обязательно, as you might guess, is related, but it has a broader connotation — meaning that something is “absolutely mandatory” or “inevitable,” but without the specific sense of an ethical/legal obligation:

Кому-нибудь обязательно делать что-нибудь.
Someone absolutely must do something.

Обязательно, что кто-нибудь будет делать что-нибудь.
Inevitably, someone is going to do something.

And finally, приходиться/прийтись can be used to mean a very strong “should” — usually in reference to something that is unpleasant, but unavoidable because of compelling external circumstances:

Кому-нибудь приходится делать что-нибудь.
Someone has no choice but to do something.

But now let’s look at what happens when the negative particle не is added to these expressions. Consider the following two English sentences:

You must speak with him.
You have to speak with him.

Essentially, they’re identical in meaning (perhaps with slight stylistic differences), and both could be translated with Тебе надо поговорить с ним. But when the English sentences are negated, they’re no longer synonyms:

You must not speak with him = “You’re not allowed” or “It would be a really bad idea”
You don’t have to speak with him. = “It’s simply unnecessary.”

And, in Russian, не надо (or нельзя) can convey the meaning “must not” or “it’s not allowed” or “it’s such a bad idea that it ought to be forbidden”. But не надо or нельзя won’t work if you mean “you don’t have to feel obliged; it’s not really necessary; you can skip doing it.” Instead, you can say:

Тебе не обязательно говорить с ним.
Ты можешь не говорить с ним.
You don’t have to speak with him.

You may have noticed that in the “positive” expression, Тебе надо поговорить с ним, the infinitive поговорить is perfective. (In fact, the imperfective говорить would be possible here, too, though it would suggest the need for a long and involved conversation; поговорить implies “have a brief word with”.)

But after negative modal expressions (either нельзя or a positive expression combined with не), more often than not you should use an imperfective infinitive. And in particular, the imperfective must be used when the meaning is “it’s forbidden, it’s not allowed, it’s immoral,” and so forth — because imperfectives, by their very nature, convey a broad and categorical meaning. (On the other hand, a perfective infinitive may be used in certain cases to convey the meaning “it’s not possible” or “it possibly won’t happen.”)

Finally, take note of where the не is placed in these two lines from Lola:

Ты можешь не звонить Карлосу.
You don’t have to call Carlos (if you’d rather not).

Ты не можешь звонить Карлосу.
You can’t call Carlos (because the phone is in the fishtank).

So, negating the verb мочь (“can; to be able”) gives the meaning “it’s not possible to do XYZ”; but negating the infinitive that comes AFTER мочь gives the meaning “it’s permissible to not do XYZ”.

P.S. If you’ve never seen a telenovela, this parody captures the flavor pretty well: