Which of the following sentences is correct?
1) If Found, Please return to
2) If Find, Please return to
Please give me reason too
In English, you can use the participles of a verb as adjectives. Using the past participle in such a way is very common:
Marry -> participle married -> John is married, John is a married man.
Write -> participle written -> The book is written, it is a well-written book.
Forget -> participle forgotten -> The song is forgotten, it is a forgotten song.
In the same way, you have:
Find -> particple found -> the key is found, it is a found key.
Your sentence is a shortened version of
if (something is) found, please return (it) to...
The infinitive (find) is not used in this way, so your second sentence is not correct.
You could, as someasw mentions, rewrite the second version into something like if you find it, please return it to....
You should use "If found, please return to..." here (I also corrected other typos).
"found" is needed because you need the passive form here: "If something is found, please return it to..."
2 is wrong the way you phrased it. Possible variation: "If you find it, return it to..."