URGENT ~ 我買左西北機票一問....

Depend where you issue your ticket. If you issue with nwa.com which was airlines wedsite. You must refer to condition when you issue ticket.
If you issue with a agency.
Ticket with -H- class price around HK$3300-3800 plus tax around 380-- non-refund // change date penalty HK$1600-2000 with advance rebook

Ticket with -M- class price around HK$4100-4600 plus tax around HK$360 -- refund for wholly un-used with advance cancel - charge HK$1800-2000 // change date penalty HK$800-1200 with advance rebook.

Ticket with -B- class price around HK$5300-5800 plus tax around HK$360 -- refund for wholly un-used with advance cancel - charge HK$1800-2000 // Free change date before each original only.

If you do not know what class your ticket holding. you can call to Northwest Airlines tel 2810-4288 to check by your eticket no. which start from 012xxxxxxxxxx . Mostly airlines will say, please check your your agency. So you can tell them you want to know your ticket can earn any mileage than they may tell what class you holding