17. "cannot…too…"结构,"cannot…too…"意为"It is impossible to overdo…"或者，即"无论怎样……也不算过分"。"not"可换用"hardly"，"scarcely"等，"too"可换用"enough"， "sufficient"等
        You cannot be too careful.
        
        18. "否定+but "结构，在否定词后面的"but"，具有"which not"，"who not"，"that not"，等等否定意义，构成前后的双重否定。可译成"没有……不是"或"……都……"等
        Nothing is so bad but it might have been worse.
        
        19. "否定+until (till)"结构，在否定词"no"，"not"，"never"，"little"，"few"，"seldom"等的后边所接用的 "until/till"，多数情况下译为"直到……才……"，"要……才……"，把否定译为肯定。
        Nobody knows what he can do till he has tried.
        
        20. "not so…but"和"not such a …but"结构，这两个结构和"否定+but"的结构差不多，不同之点是这两个结构中的"but"是含有"that…not"意味的连续词，表示程度。可译为"还没有……到不能做……的程度"，"并不是……不……"，"无论怎样……也不是不能……"等。
        He is not so sick but he can come to school.