1. 从词义上看：look 指视觉印象；appear 指外表给人的印象；这两者可能是真象也可能是假象，appear 的不确定性更大一些。seem 侧重指根据某种迹象作出的推断，也不一定是事实。
(1) 三者均可后接不定式，但 look 之后一般只限于 to be (且较少见)。如：
正：He seems [appears，looks] to be tired. 他好像很累了。
正：He seems [appears] to have travelled a lot. 他似乎去过不少地方。
误：He looks to have travelled a lot.
(2) look, seem 之后可接介词 like，但 appear 之后一般不能。如：
He looks [seems] like a fool. 他看起来像个大傻瓜。
(3) appear 和 seem 不用于进行时态，但 look 有时可这样用。如：
正：He looks [is looking] well. 他看上去气色不错。
误：He is seeming [appearing] well.
(4) 三者均可用于 it 开头的句子，且三者之后均可接以 as if 或 as though 引导的从句，另外 appear 和 seem 之后还可接 that 引导的从句。如：
It looks [seems，appears] as if he has lost interest in his job. 看来他对自己的工作已失去了兴趣。
It seems [appears] that he is very tired. 他似乎很累了。
注：后接 that 从句时，有时可用 so，not 代之。如：
A：Are they reliable? 他们可靠吗?
B：It seems [appears] so (not). 似乎是可靠的(不可靠)。
It seems [appears] that he is ill.＝He seems [appears] to be ill. 他似乎病了。