FM in a trend forecast means "FROM", and that denotes the time at which the change in Wx would take place. FM here is usually mentioned with "TL" which is "UNTIL". In TAF, FM is different in which it denotes a quick change that will take place in less than 60 minutes during the prevailing weather conditions in a certain time frame.
BECMG, as you may guess, stands for "BECOMING", and it denotes a permanent change in the prevailing Wx conditions. If the term BECMG is followed by a time frame, for instance BECMG 0710, then it's self-explanatory i.e between 7 and 10. If there is no time frame for the change in the Wx condition, you may expect that change to happen over the next 2 hours after the last Wx reported.
Good luck.