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。