我没买到那本书。[I tried to by that book, but] I could't buy it. [because they were sold all in the period of one day] 大卫没找到玛丽[David has gone to find Mary,but] David didn't find Mary 我没看到他I didn't see him. 我没看他I didn't look at him

In Chinese,there are two negative word :不 and 没。不 express Subjective view and 没express Objective fact .

Yes, "没" is a negative word in the two sentences. And it is a primary negative word in Chinese sentences.

Of course , 没 can't be omitted

到 in these two cases express the result. So if you omit 到，these two sentence mean: I didn't buy that book. David didn't look for Mary. And if with 到，that means, I went to buy but couldn't get that book. David couldn't find Mary .