by turns

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。

1. He gets cheerful and depressed by turns.
   他的情绪高一阵低一阵。
2. We did the work by turns.
   我们是轮流做这项工作的。
3. The shop assistants had to have their days off by turns.
   店员要轮流休息。