Mainly it depends on "jukugo*" or not. There're many exceptions though. "Jukugo" means the word consist of 2 kanjis and is read in "on". e.g read in "on" noun 質問 shitsu-mon : question 回答 kai-tou : answer verb = noun+suru 質問する shitsu-mon suru : to ask 回答する kai-tou suru : to answer e.g read in "kun" 問う to-u : to ask 答える kota-eru : to answer This is very difficult question. Actually I don't know how I choose on or kun. Maybe I memorize them...