from time to time

不时的，常常

frivolous, frivolously, frizz, frizzle, frizzy, fro, frock, frock coat, frock-coat, frog, frog-march, frog-spawn, frogman, frolic, frolicsome, from, from memory, from now on, from side to side, from then on, from...to, frond, front, front man, front runner, front-page, frontage, frontal, frontier, frontiersman, frontispiece, frost, frost-bite, frost-bitten, frostbound, frostily, frostiness, frosting, frosty, froth,