About mirroring

hi, I am new at this and yes, it is driving me crazy, but I am learning.  I remember the 90s.

Could you be a bit more precise as to what you mean, please? With Screenshots if possible.

In order to know how to fix things, it would be good to know, what is wrong with the things you mirror. 

And how are you mirroring. are you using a Mirror Modifier? 

let us say you mirror. there is a line not straight or then one I delete the line or the face or the or the vertices. I go back to and do a push a f key. it goes to other lines or vertices.