Is expiation obligatory upon one who swore that a person did not understand him, and then it became clear that what was understood was part of what was in his intention?

As long as you are truthful in your oath that your mother did not understand your point of view in a particular discussion, then there is nothing upon you. However, if you took that oath and it became clear that she did understand, then you must offer expiation for breaking an oath (kaffarat yamin). And if you knew that she understood and you swore that she did not understand, then this is a false oath (yamin ghamus) which necessitates repentance, and there is no expiation for it according to the majority of scholars.