Trim two

When trying to trim using two entities, the first entity is selected, no problem. When I select the second entity, almost always a line in my work, both entities "lengthen".
BTW, when i try to use an entity that is not a line, say a rectangle, or ellipses the entity to be trimmed doesn't trim within the entity. The entire line, for example, is trimmed from one side of the rectangle, ellipses, whatever. I would think the line would be trimmed within the entity I selected. I have to basically draw non continuous lines and trim twice.
Any thoughts are appreciated.