Why do my javascript and jquery source files consistently not load?

I quite frankly run into this problem every time I try to code html or anything web related.

<script type="text/javascript" src="/js/jquery.js"></script>

Sometimes it works, sometimes it doesn't. I try a hard refresh, doesn't work. I try changing the file name sometimes. It may work for a few. I try a new page, it doesn't work. I'm really lost on what to do here. If I create a /js folder, and stick my freaking js files in it, what can I do so that it will pick up on them every time, without fail? This isn't related to other coding mistakes, because I'm taking code directly from working examples, and they do not work because of it not picking up on the jquery source file.