Oct 23rd 2008 Shinn
in=中にいる I'm inside the store. 店の中にいる。I'm in the restaurant. レストランの中にいる。
at=にいる　I'm at home. 家にいる。I'm at work.仕事にいる。I'm at the store. 店にいる。
for=のため I'm doing it for you. 君のためにしている。For justice! 正義のために!（笑）
from=から Where are you from? どこから来ましたか？

Oct 23rd 2008 nunka
In and at are easy to mix up, because sometimes they're both okay.

たとえば、レストランの中にいる。

I'm in the restaurant.
I'm at the restaurant.

Either one is good. So... sometimes "at" means "in," but I don't think "in" ever means "at" :)

Nov 11th 2008 Ryantaku
I also would like to find out the different between "of" and '"for". Please let me know.

Nov 13th 2008 Digger
"Of" is similar to "no" in Japanese. It indicates a part of something.

Examples
1. the belly of the dog ---> (I petted the belly of the dog.)
2. the teacher of the class ----> (I like the teacher of the class.)
3. the sound of music ---> (I heard the sound of music).

"Of" can also mean "like" or "similar to"

Example:
1. The shoe smelled of stinky feet.

"For" can indicate the purpose of an action.
Examples:

1. I did homework for class.
2. I looked for my keys.
3. I bought flowers for my girlfriend.
4. I jumped for joy.


"For" can also indicate the purpose of a thing.
1. The movie is for kids.
2. The pencil is for writing.
3. The carrots are for salad.

It can also explain the reason for a behavior. But this is not common.
Examples:
1. I cried, for someone ate my cookie. ("I cried because someone ate my cookie." is more common.)

Sometimes "for" also indicates the direction of an action.
1. I reached for the gun.
Strangely, people don't say, "I walked for the store". They say, "I walked to the store." But they do say, "In the race, I ran for the finish line," or "I made a run for the tree before the bear could eat me."

I hope this helped!