from your example, it looks like you're not including the rest of the text - just the first one or two characters.
In that case, you'll need to include the Right and Length functions as well. Or, consider Lower(LeftWords( ... and RightWords (and WordCount)..
If you field have any information, then do you want to have first letter as lower case? Your calculations results is when First letter is empty (then you should check if all field is empty) then show first two letter as lower case. Second option if first letter is not empty then show first letter as lower case. I think what you want is if field is empty, then show nothing, but if it have information give me the first letter as lower case.
I would to you calculation like this
text = Sentence1
// If Sentence1 Not is empty, then give me the first letter as lower case
text ≠ ""; Lower( Left( text ; 1 ) )
Left ( Sentence1 ; 1 ) ≠ "\"" ;
Replace ( Sentence1 ; 0 ; Length ( LeftWords ( Sentence1 ; 1 ) ) ; Lower ( LeftWords ( Sentence1 ; 1 ) ) ) ;
Let ( n = Case ( Left ( Sentence1; 1 ) = "\"" ; 2 ; 1 ) ; Replace ( Sentence ; 1 ; n ; Lower ( Left ( Sentence ; n ) ) ) )