When does a convex lens behhave as a simple microscope and what are the conditions for the object and the lens to act if there are any?Is the magnifying power of microscope fixed beyond which it will act as a regular convex lens
Have you tried googling "Magnifying glass"? It's all a matter of where the object is placed whether you get an inverted real image or an upright virtual image*. The first magnifying lenses were, indeed, all they had as a microscope. But to get strong magnification that way, you need a tiny objective and it kills your eyes. Using a second (eyepiece) lens makes things a lot easier.
*A convex lens can give magnification in both cases but, used as projector, it produces a dim, magnified image. (Compare the brightness of a projector lamp and the image as it appears on the screen).