详细解释:轮流, 交替
词意辨析：
by turns, in turn这两个词组都与turn有关。
by turns: 意思是“轮流地”、“交替地”、“忽而...忽而...”（ one after another； alternately or in rotation）we read the poem by turns.
我们轮流读这首诗。they sang songs by turns.
他们轮流唱歌。he grew pale and red by turns.
他的脸一会儿红一会儿白。
in turn: 意思是“依次相继地”（each in due succession）we sang the song in turn.
我们依照次序唱歌。we were summoned in turn to see the judge.
法官依次召见我们。例句：
He gets cheerful and depressed by turns.他的情绪高一阵低一阵。