1.He (can't/may not) walk so quickly cause his leg is still aching.2.There was not a single cloud in the sky,and we (might/were able to) see the stars.3.He (need/may) arrive on the next train.4.He (may/has to) light a fire,it is becoming too dark.5.He (is to/can) return to Paris tomorrow in order not to be late.