the letter a

Might be me, but the top left segment doesn't seem to be logical.

Hint if it's not: Follow the lines to match up, there's a curling flick in the very top left.

Not logical at all. please help more!

You can guess c5r17, c1r2, c5r1, because the false value leads to a dead end. But after that you can only try both values e.g. for c5r3 etc.

Could someone please give a hint for the top left corner?

1st 2 is in R1,C1.
Then from the end of the 8 in R1 looks like a "C" or hook shape.
Hope that helps :-)

Thank you Sparrow! had tried too many possibilities :)