by turns的意思是“轮流地”、“交替地”、“忽而…忽而…”( one after another; alternately or in rotation);in turn的意思是“依次相继地”(each in due succession)。例：

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。

We sang the song in turn. 我们依照次序唱歌。

We were summoned in turn to see the judge. 法宫依次召见我们。

【注意】跟by turns同义是 take turns，但二者语法不同。前者是副词片语，作状语用，后者为动词片语。例：

The classes visited the exhibition by turns.

The classes took turns to visit the exhibition. 各班学生轮流参观展览。

【注意】请注意，on the turn的意思则是“正在变化(转变)中”。例如：

The bullish stock market is on the turn. 股票牛市开始变了。