I was reading about this subject in a grammar book and it was written :

So, I was wondering if it is always like that , I mean , if I want to say that I need strongly to to do sth  I should use must , like in I must study hard , I can't say I have to study hard , since this decision  was  made by me , the speaker . On the other hand , if I someone tells me : you need strongly  to study hard , it would be you have to study hard and it is wrong to say you must study hard , since the someone else has made the decision .  Is that the way we can make differences between must and have to ? Did I catch the book explanation properly ?

Thanks ,