How does "следует" behave towards "должен", "надо" and "нужно"

Question

There is this question about надо and нужно, but how do

• следует,
• должен,
• надо and
• нужно

behave towards each other? For instance

Тебе следует купить новую лопату.

How would this sentence change, using the other verbs and what would be the order with respect to the strength of the order. Maybe there are also very common alternatives to these four verbs / cases. (The case Тебе нужна новая лопата can be omitted.)

Answer 1

"Должен" (one must, one has to) is the stongest one, then comes "следует" (strongly recommended) and then "нужно" and "надо". "Нужно" and "надо" (it is necessary) are synonyms, but I would say that "нужно" is slightly more formal and more used in the written speech.

As for the grammar, "должен" is used with the Nominative case:

• Ты должен купить лопату

But "следует", "нужно" and "надо" are used with the Dative case.

• Тебе следует купить лопату.
• Тебе надо купить лопату.
• Тебе нужно купить лопату.

All of them can be used both with the perfective and the imperfective infinitive depending on the situation.

• Ты должен купить фрукты в этом супермаркете. You have to buy fruit in this supermarket now.
• Ты должен покупать фрукты в этом супермаркете. You always have t buy fruit in this supermarket.

I would also add "обязан" to this list. It is used with the Nominative case and means "it is obligatory, compulsory). It is even stonger than "должен".

• Ты обязан купить лопату.