1. be表示“成为”时，多用于将来时、祈使句或不定式。如：
I would like to be a bus-driver.
My younger sister wants to be a movie star.
2. become多指身份、职位等的变化，强调变化的过程已经完成，后面可接名词或形容词。如：
Later the boy became an artist.
Her mother became angry when she heard the news.
3. get多用于口语，表示一种变化过程，强调的是“渐渐变得”，后常接形容词的比较级。如：
It's getting darker and darker outside.
In winter the days get shorter.
4. turn指在颜色和性质等方面与以前的完全不同，强调变化的结果。如：
Leaves turned brown in the mountains.
