Are You Taking Advantage of the Arrow Keys?
Here is an important helpful aid in modeling in 3D. Since it can be challenging to stay on axis, SketchUp has some handy ways to help us. There are four Arrow Keys on the computer keyboard -up, down, right, and left. If you need to draw a line on the red axis, click the mouse to start the line, then tap the Right Arrow Key. Note that no matter which direction you move your mouse the line is forced to follow the red axis. To force the line on the green axis, tap the Left Arrow Key. And to force the line on the blue axis, tap either the UP or Down Arrow Key.
These Arrow Keys work also with the Move/Copy Tool. So to force movement of a copy or component on the red axis, tap the Right Arrow Key. And similarly use the other arrow keys to force movement on the other axes.
As your model becomes bigger and more busy with multiple components, these Arrow Keys become a necessary feature to keep things moving and organized on the axes.
SketchUp provides another way of forcing the action along a specific axis using the Shift Key. In this case, you need to start the line or the movement on the axis of choice, then hold down the Shift Key. This will cause the line or movement to stay on that selected axis. So the Shift Key will work with any of the axes, but it is only depressed after the action is already going along the selected axis.