as though,even though,though

as though（＝as if），意为“好像；似乎”，引导方式状语从句或表语从句。如： 
He spoke as though（as if）he had been here before．他说话的口气好像他以前来过这里。 
It looks as if（as though）it is going to rain．看起来好像要下雨。 

even though（＝even if），意为“即使”，引导让步状语从句。
though也引导让步状语从句，意为“虽然”；
even though有退一步设想的意味，与though不同。
though引导的句子所说的是事实，even though引导的句子所说的则不一定是事实。

例如： He will not tell the secret even though（even if）he knows it．即使他知道这个秘密，他也不肯说出来。 
He will not tell the secret though he knows it．他虽然知道这个秘密，但他不会说出来。