This can be very confusing when learning English. Here is a video that may help. http://www.engvid.com/english-grammar-to-for/

1) to 2) for 3) to

私は日本語がまだ下手ですが。。 I can try to relate those two to explain things easier Probably 'to me' would be like 私に、for example, He gave the book to me (彼がこの本を私にあげました) 'for me` would be something like 私のために He washed my clothes for me (彼が私のために、私の服を洗ってくれた) Hope it helps you to understand a bit better

You wouldn't use either of those words in those ways. I can re-word those statements for you so that it will sound more natural. 1)You are my best friend. - the word "my" is what's known as a "possessive", and is the possessive of the word "me". A possessive indicates ownership, origin, or possession (to have). 2)Water is essential. - While it's not grammatically incorrect to say "water is essential to/for us" (both are technically correct), it's not necessary to say who water is essential to, because water is essential to everyone. This sort of statement isn't something that anyone would ever need to say, unless an alien who didn't need water came down from space... :p` 3)This is impressive. - It isn't necessary to say "to me" because it is implied that you are impressed when you are the one saying it. Alternatively, you could have also said "that is impressive". I hope this has helped you! :)