[误] The lake is two meters under sea level.
[正] The lake is two meters below sea level.
[析] under的意思是在某物的下面，而below=lower than, 即"低于"。
[误] Under the help of our teacher, all of us passed the exam.
[正] With the help of our teacher, all of us passed the exam.
[误] I think it is diffcult to make mself to understand.
[正] I think it is diffcult to make myself understood.
[误] I am understanding the lesson now.
[正] I understand the lesson now.
[析] understand一词没有进行时态。类似的词还有belong, find, hear, love, like等。