Intermission: Keigo and some archaic speech
Keigo is a prescribed way of using japanese that encodes information about respect, humility, and politeness. Some of the politeness mechanisms of normal japanese (like ます and です) overlap with keigo, but keigo goes beyond that.
Keigo makes strict distinctions between politeness, respectfulness, and humility, and also adds mechanisms for "word beautification". We'll cover the normal ways to do some of this later on, but keigo is a level above what the normal language does.
In keigo, certain phrases act as alternatives to normal words, and they have different levels of respectfulness, humility, politeness, beauty, etc. Because this is prescribed, not all japanese speakers understand it 100%, but everyone's aware of the basics and the fact that it exists.
These phrases basically act like euphemisms for normal words. These phrases are usually longer than what they replace (since longer = more formal), and since they're basically euphemisms, they're kind of nonliteral/indirect. If you know that keigo exists, you'll be less confused when you run into them.
Here's an example of the keigo versions of the word する "do":
なさる (respectful) いたす (humble) します (polite)
Keigo cares about your social status compared to other people, which is why there's several categories and why it's prescribed.
Keigo is basically a register of speech. Just like someone living in a slum would be totally out of touch with the way the language used in a cathedral, someone who learned japanese playing video games would be mostly out of touch if they ended up in a business meeting. Of course, a game will probably use keigo, but not in the right ways for you to acquire it.
On the subject of registers of speech, you will occasionally run into things like よりて where よって normally goes, using the unreduced version of the stem described in Lesson 12, but this is basically exclusive to intentionally archaic writing.