That certain thing you just can´t learn!

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 Why is it that there is that certain thing in both the languages I am learning that I just can´t learn! It doesn´t matter how often I hear it, how often I am corrected, how well I know it´s wrong...I still can´t do it right!
Do you have this?
What is it you can´t fix/learn? Maybe someone here will have a tip for you :)