In Russian, understanding some special verbs—called modal verbs—is really important. These verbs help us talk about strong needs in different situations. The verbs "должен" (must), "следует" (should), and "можно" (may) let us express different levels of duty, permission, and possibility.

1. Strong Need

When we talk about something really important that you must do, we usually use "должен." This word shows a strong obligation or need, whether it's real or imagined.

Example Sentences:

• Если ты хочешь сдать экзамен, ты должен хорошо подготовиться.
  (If you want to pass the exam, you must prepare well.)
• Если бы он пришёл вовремя, он бы должен был увидеть её.
  (If he had come on time, he should have seen her.)

2. Imaginary Situations

In imaginary situations, we use a special way to talk about things that should have happened but didn't. This shows we are guessing or feeling sorry.

Structure:

• Если бы + субъект + должен был бы + глагол (infinitive)

Example Sentences:

• Если бы у меня была работа, я должен был бы больше зарабатывать.
  (If I had a job, I should have earned more.)
• Если бы они знали о встрече, они должны были бы прийти.
  (If they had known about the meeting, they should have come.)

3. Should and Ought To

For giving advice or suggestions, we use "следует." This shows a strong recommendation, but in a nicer way. It’s especially useful for imaginary situations.

Example Sentences:

• Если бы ты занимался регулярно, тебе следовало бы добиться успеха.
  (If you had practiced regularly, you should have succeeded.)
• Если она хочет улучшить свои навыки, ей следует читать больше книг.
  (If she wants to improve her skills, she should read more books.)

4. Possibility and Permission

The word "можно" shows permission. In situations where specific conditions matter, it means that something could happen if the right things are in place.

Example Sentences:

• Если погода будет хорошей, можно будет пойти на пикник.
  (If the weather is good, we may go on a picnic.)
• Если бы они пришли вовремя, можно было бы обговорить все детали.
  (If they had come on time, we could have discussed all the details.)

Summary of Usage

1. Должен is for strong needs in real and imaginary situations.
2. Следует gives softer advice, especially in imagined situations.
3. Можно shows permission, which can link to necessity in certain situations.

By understanding these modal verbs, you can express needs and obligations much better in Russian. This helps you speak and write more clearly. Remember, the context matters a lot when choosing which verb to use because it can change what your sentences mean.