shall, must, should
Shall-Used to indicate a future action. I have seen a translation as "it shall be"---deberá.
Must-Indicating that something needs to be done. I think it is translated as deber. I must finish my homework.
Should-Used to indicate obligation. It is like "must" but it is not as strong as "must"
I should do my chores.
I must stop the dictator.
I shall become president someday.