There is so much we can work on when mastering Freestyle. One aspect of the stroke that causes a lot of swimmers trouble is head position. Some swimmers tilt their heads up slightly either when they breathe or when their heads are in the water during their strokes. Some swimmers move their heads from side to side, never really finding that perfect alignment position. If any of my old coaches are reading the previous statement, they are all nodding along as I used to be extremely guilty of that one.
I once had a coach tell me that the best way to swim is to imagine yourself as twice the size you are. Over my career as both an athlete and coach, I've seen a lot of swimmers who swim like individuals half their size. What I mean is that they shorten their strokes, especially in Freestyle or as some call it Front Crawl, which causes them to lose out on some much efficiency in the water. One big idea that I try to get swimmers to understand is the notion of swimming like someone twice their size.