... certaintyMary isn’t in class. She must be sick(present only)Mary must have been sick yesterday. Have to (1) necessityI have to go to class todayI had to go to class yesterday21 (2)lack ... gap with must , have to , must not and do not have to. Table 2.3 above shows that students had a lot of difficulty with have to and must . It seems not to be easy for student to distinguish ... Modal verb "phải '' in Vietnam may be close in meaning to must, should, ought to , in English . and modal" có thể " close to can, may, could, might. According to Diep...