What is the difference between include() & require() ?
Both functions are used to include specific files to your current page, however require will throw a fatal error if the referenced file does not exist or does not load. Include() will allow for the next line to execute and continue.
What is the difference between body onload() and document.ready() functions?
There are actually (2) differences. The first one being that document.ready() can be used multiple times on the same page, body onload() can only be used once. The second difference is that document.ready() is called as soon as DOM is loaded, body onload() is called after everything on the page is loaded, which would include DOM, images, resources etc.
Why is it important to understand Software Engineering if you already know how to write "Code"?
Writing "code" is only a piece of the puzzle when it comes to the real world. Writing code is an important piece to any software project, however understanding all elements of requirements, design, integration and impact on other systems are equally important when building or updating a software system in industry.