Using present simple on story telling?

[Aikuzo]

Say, I was talking with my friends and I was telling them a story about what happened yesterday, then I said: "You know what that guy did? He counts the coins and throws it on the river!"

Since I'm telling a story about yesterday, why should not it be "You know what that guy did? He counted the coins and thrown it on the river!"
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I quit studying English years ago and have just come back to study the basics.

[Cerberus™]

Both are possible. The past simple is regular; present simple is more vivid: you are, in a way, putting the listener and yourself in the perspective of someone who is there right when it happens. It is called the historical present. In practice both tenses are frequently used in the same kind of situation; you are free to choose.
However, there are some, like me, who will use the historical present sparingly, so as not to dull its vividness.

You probably meant "threw them" instead of 'thrown it".