1. The letter on the table is yours.
2. A letter on the table is yours. Find it.
[#1 is correct, isn't it? What about #2? If there are three letters, and one of them is yours, then can we use #2?]
yes, yes, sure you may
by the way
'You may do it' means you are allowed to do it. (no law against it)
'You can do it' means you are able to do it. (strong enough for example)
In #1, the sentence states that there is only one letter on the table, and it is yours. This implies that there is no need to search for the letter on the table since it has already been identified as yours.
In #2, the sentence suggests that there are multiple letters on the table and one of them is yours. In this case, it would be appropriate to search for the letter to identify which one belongs to you.
So, to determine if #2 is correct or not, it depends on the context and the actual number of letters on the table. If there is only one letter on the table, then #2 would not be applicable. However, if there are multiple letters, then #2 would be the appropriate approach to find the letter that belongs to you.