1．Be sure +不定式用于祈使句,作“务必、一定”讲.
Be sure to come to my house.
Be sure to finish your homework as soon as possible.
2. be sure+从句,表“确信、对…….有把握”
I am sure he is coming. = He is sure to come.
3. make sure +从句 = make sure of 弄清、查清楚
Make sure that he has handed his workbook.
Make sure of it before you start out.