A realistic drawing has to be three-dimensional.
To get a feeling for plasticity and three-dimensional space, start with drawing simple forms. Once you have mastered those you can apply what you have learned to draw more complex subjects/objects. Here are some easy examples:
Example 1: Ball
This is a free hand drawing and not a mathematically correct alalyses of a ball!
(1) At first I draw the lines - here extra strong. The small lines on top, bottom and the sides hint that the perspective is frontal. Imagine a cross running throgh the circle. The vertical shading lines, and the horizontal lines, have the same beginning and the same ending. The object is round so of course the strokes have to be applied accordingly and are arch-shaped.
(2) In the second sketch I added more shading with rough strokes so you can still see the lines.
(3) In the end the lines are hardly visible anymore. The lines are extremely strong and slightly blended.
Once you have done that you can work with different lighting and perspectives.
Here are various examples of different shades. Be aware that the alignment of the pencil strokes differs with different light.
Example 2: Cup / Glass
You can think of the form as a rectangular at first. All the lines are parallel. The circle on top and the circle on bottom are in one line and have the same shape (oval because of the perspective). In the second sketch the cup is roughly shaded. To make the drawing more realistic I also added light and shading in the third sketch.
Example for other forms: Pencils
Another exapmle for a simple form is a pencil. Here you can see two pencils drawn from a slightly differnt angle. Whereas the end of the pencil is visible in the first sketch and drawn three-dimensional, the pencil top can't be seen in the second sketch. Also the lines of the shading are different due to the different perspective.
Especially if you have problems with three dimensional space, it's good to train the eye by drawing simple forms from different angles. Once this comes easy for you, you can start with more complex objects and subjects.