[译] 他摘下一根草，放进嘴里。
[误] He picked a grass and put it in his mouth.

[正] He picked a blade of grass and put it in his mouth.
[注] 由于 grass 是不可数名词，表示“一根草”，不能说 a grass，应该说 a blade of grass 或 a leaf of grass。