a是个词根,有三种意思:1. 以某种状态或方式,如: ablaze, afire, aflame, alight, aloud, alive, afloat等2. at, in, on, to sth,如: abed, aboard, abroad, ahead, afoot, ashore等3. not/without, 如: amoral, asexual, atypical等 船的甲板和车厢的地板是板(board),所以aboard是上板(车.船).board可做动词,登机口的工作人员会喊…