You go first,if I haven't come at 5:00 pm./If I don't come until 5:00 pm,Don't need to wait for me./No need to wait for me.
If I don't come back home before/at 8 o'clock in the evening,you eat first,please.
If you father don't come back home by 10:00 pm,remember to give him a call/send him a short message.
Hope you will reply my mail as soon as possible upon receiving my mail/letter.
you'd better write back to Jack/You'd better write a letter back to Jack as soon as possible.
You'd better return a letter back to Jack.
Please reply Jack by email/ short message/ telephone as soon as possible.