"I saw someone cry!' - why "cry" and not "crying"?

I need a book or a trustworthy source of information on the internet that has detailed information about the tenses, English grammar, the basic of the English language and the properties of English.
Hi Picnic:

Here are two good free Internet references on English grammar:

http://owl.english.purdue.edu/owl/section/1/5 /
thank you! ill be revweing it soon

what about my single question?
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"I saw someone cry" and "I saw someone crying" are both possible. There's a very slight difference of emphasis between them, but it's really hard to explain. It seems to me that somehow the first focuses more on the specific instance of crying, and the second is more descriptive of the whole situation.
The best way I can explain the difference is that the -ing form is continuous and refers to the duration of an action. The base verb is used when referring to a discrete event.

I saw someone crying. (focuses on the duration of crying for some time. I saw someone in this state.)

These focus on a one-time event, not the duration:
I heard a baby cry in the night. It woke me up.

Once I saw my mother cry. I remember it vividly.

I heard him telling a lie. (I was there and heard him during the time he was lying.)
I heard him tell a lie. (It happened at least once, so I can conclude that he does not always tell the truth.)
Thanks, A-Stars -- that's a good explanation!
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I saw someone cry. I saw someone experience an (entire) episode of crying, from beginning to end. Compare: Someone cried. I saw it happen.

I saw someone crying. I saw someone during the time when they were crying. At the time I looked, that person was in the act of crying.

Compare: Someone was crying at the time I looked at them.


Both forms are correct. But generally If we say' I saw someone cry' , That means I saw the whole action of crying .but if we say ' I saw someone crying' , that means I saw only a part of the whole action. So we normally say I saw someone crying. Any comment on this is appreciated