didnt 是对任何人称过去时的否定。
doesnt是对第三人称现在时的否定，也可以是对喜好，习惯的否定。
例：
Eyesight doesnt get any better at my time of life.
到我这把年纪，视力绝不会变好的。
We didn't have much time.
我们没多少时间。
She didn't do a full day's work.
她并没干一整天的工作。
God help us if this doesn't work.
如果这个行不通，那就要靠上帝了。
She doesn't understand the difference between right and wrong.
她不能明辨是非。
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