Random/leading space in paragraph text overflow

OK.

First off, I admire you for trying to learn this on your own (I'm largely self-taught in a number of fields where I made a living, including InDesign before there was a lot of training available, but I had a lot of experience with other layout apps), but InDesign is very complex and far from intuitive in may areas and some basic training would be very helpful for you.

InDesign is all about styles -- Paragraph Styles and Character Styles, primarily, and all text carries a paragraph style by default, Basic Paragraph, unless you change that. Your screen captures are helpful, but they are not showing the style information, so I don't know for sure what's going on. It does look, however, as if you have a very large value assigned for leading (the space between lines) as well as some paragraph spacing (additional space separating paragraphs) which could be included as space before or space after as part of a style, or as space between paragraphs using the same style. At the moment, my best guess for the  gap at the bottom of the page with the single line at the top of the next page is that the paragraph has a Keep With Next  (n) Lines enabled and due to leading and/or paragraph spacing there is not enough room in the frame for the last line and any text from the next paragraph. Your text is not set to align to the grid, and your vertical justification is not set to justified, which would have spread the text evenly to fill the height of the text frame. Vertical Justification is a text frame property, not a text property, and has nothing to do with how your text is aligned horizonatally.

As I said, InDesign is style-driven, and all of these things can, and should, be controlled by properly defining styles for the various types of paragraphs in your document. The bad news is that all of these things can also be set outside the style by selecting text and changing the setting values in the Paragraph, Character, or Control Panels, creating what are called local overrrides or local formatting, and I suspect that's what has happened in your document. I don't mean to insult you here, but your page has some tell-tale signs of someone used to working in Word without using styles -- empty paragraphs to get space between paragraphs being the most obvious. Fixing a file that is built that way is a very time-consuming project.