A-verb + nakereba naranai desu (なければ ならない です) – Must do (verb)
Example:
Watashi-wa oyoga-nakereba naranai desu (わたしは およがなければならない です) – I swim must do/have to – I must swim
My girlfriend also tells me that -shinakereba ikemasen is also acceptable and has the same meaning. Its a bit less of a tongue twister.