move vertices along edge extensions
If you want to move the vertex while keeping the plane,Constrain Vertex Movement to Edgesfunction is enabled.
First select the vertex you want to move.
G twiceBy pressing down, the movement of vertices is restricted on the edge.
In that state CPress to change the movement range toExpand to the extension line of the edgewill be
In architectural modeling, this kind of axis fixation is often used, so it is convenient to remember.