By accident I booleaned and stuck another object to this one. I couldn't separate them so I deleted vertices, edges and faces few at a time and managed to get rid of the object. but i think that yellow lines are base of the object joined. I can't get rid of them. With the mouse i can point the vertices and sides. i can even make a face but can not delete at all. Please help me with this. Thanks.
Ctrl + Tab is your friend here. Got into edit mode and switch to Vertex selection. Select the vertices you want to delete, hit x, and select "Delete Vertex." Delete has several options, and some of them are selective. My guess is that you have been selected "Delete only faces" or something like that.
Another trick: select 1 vertex of the item you want to remove and hit the L key, which will selected all linked geometry. Usually it'll select the whole object you're wanting to delete.
Also make sure that you've applied or removed any modifiers when you have mesh geometry issues like this—will save you a headache later on.
