Thread: мне надо vs. я должен - вопрос

Hi,
Can anybody give me rules when to use мне надо and when to use я должен?
Например, I can say:
1. Мне надо (сейчас) пойти на почту.
2. Я часто должен поехать в москву.

It is wrong to say (right?):
Мне часто надо поехать в москву.

But it is okay to say:
Я должен (сейчас) пойти на почту.

Can anyone explain this to me / give me rules?

Спасибо.

"Мне надо" means that "I have a need to do something". For example: "мне надо поесть", "мне надо поспать", etc. "Я должен" is more like "I am obliged" or "I have to do something". For example: "я должен идти на работу", "я должен вернуть долг". "Быть должным" also means to owe something to someone: "я должен другу 100 рублей".
This rule isn't strict, though, and quite often these two words can be used interchangeably.
It's useful to remember, though, that the word "надо" is derived from "надобность" (need), whether "должен" is derived from "долг" (debt).

It is wrong to say (right?):
Мне часто надо поехать в москву.

It's wrong to say not because of the use of the word "надо" (which is perfectly OK) but because of the use of the verb "поехать" which is the perfect aspect verb and because of that cannot be used with the word "часто".
So the right way to say is: "Мне надо часто ездить в Москву".
You, of course, can say: "Я должен часто ездить в Москву".

But it is okay to say:
Я должен (сейчас) пойти на почту.

Yes, it's OK, but you also can say: "мне сейчас надо пойти на почту". At least there is no any grammatical mistake in this sentence, all the difference is about the modality of the action: whether it is dictated by the need or obligation of some kind.

rule of thumb: надо - need due to circumstances, because I need to, I want to; должен - must, obligation, strict rule. This is the principle, but it may be complicated sometimes.

Спасибо.