Usage of shall and should
"I thought I should tell you the results."
I think that according to tense agreement, shall became should there and I was wondering, then, if shall has the meaning of what is the right or sensible thing to do or duty? Then can I also say, "You shall brush your teeth before going to bed." for the same meaning as "You should brush your teeth before going to bed." Or only should is used for the meaning of what is the right or sensible thing to do and tense agreement in sentences like the example, but shall does not have the meaning of duty? Thank you so much as always and have a good day.
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