That is a question that requires a very convoluted and complex answer, because modeling just works in a certain way. Polygons behave in a manner that follows rules and for that you need to learn a lot about poly modeling. As you progress and you start making mistakes and see what works and what doesn't, you then get an intuitive sense of what not to do. You can do things that look like it worked until you need to do some other thing only to realize you made a big mistake, because now you lost all control of your mesh. But the short answer is no, you can't just do things that in out brains would seem like the most normal thing to do. You can watch the Fundamental courses that go into this topics here:
https://cgcookie.com/courses/blender-3-basics-intro-to-blender
https://cgcookie.com/courses/fundamentals-of-3d-mesh-modeling-in-blender
https://cgcookie.com/courses/blender-mesh-modeling-bootcamp
https://cgcookie.com/courses/pothead-create-a-hard-surface-character-in-blender
