SECTION TWO: ABSOLUTE TERRITORY
At this point, you should begin reading, skimming later lessons to see what the guide has to say about the unfamiliar grammar you encounter. The remainder of this guide is half guidance, half reference. Because it's half reference, I'm going to use jargon more often. Be ready for it.
Language learning takes time, even if you put in a lot of effort. You have to apply yourself, but you won't see big results until you do it for a long enough time. Think about what that means. The very first time you start reading, everything is going to feel really weird and you're not going to "get it". That's okay. It'd be weird if you didn't react that way. But reading (or using other forms of input) is the only way to make that weird feeling go away.
And this goes for every little part of reading, with new words, grammar patterns, ways that japanese people communicate, and figures of speech all feeling weird and alien at first and not making sense until you're exposed to them enough, even if you study them a lot without really using what you've started learning. That's why you've gotta start reading. You have to start the process of making your brain familiar with it in the ways that we don't understand yet.
Here's another way to think about it. Let me take a moment to preach. This guide can't teach you japanese. Nobody can teach you japanese. The only way to learn japanese is to understand messages that are written or said in japanese. Why is the focus on "understanding messages" in particular? Two reasons:
One: If something is way out of your depth, you won't learn anything from it. If you don't understand what's being said, you won't learn anything new about the vocabulary and grammar used in it.
Two: It's totally possible to consume something and understand its individual parts but have no idea what it "means". You can break 食べてきました down into its individual parts, and that helps you see the syntax, but unless you understand what it's saying, not just how it's said, you won't acquire anything from the message. Understanding the content or meaning of the message itself is a precondition to learning from it.
A lot of people start learning a language and focus a lot on picking apart messages one word or phrase at a time, like it's a puzzle. This can make things seem less confusing, but if you insist on doing it all the time, you'll miss the big picture and fail to comprehend a lot of messages.
This is made worse when you think that your grammar guide teaches you everything nonliteral that you need to know. In reality, guides can't teach you all the high level stuff unless you're some kind of linguistics savant. Even if you know everything your guide explicitly teaches you, the guide will always leave out important background information about why things work how they do, things that would make other basic grammar way easier to understand.
Once you figure out "Huh, 食べてきました seems to be used in <situation X> a lot", that's when it's time to break it down and try to see what individual parts contribute, comparing them to similar phrases you already know. If you always start with the individual parts, you'll miss the general idea, and fail to associate the phrase with the situation.
That association is what lets your brain acquire the phrase and get a fluent handle on it. Noticing what things mean when they're used is the thing that you need to do if you want to learn japanese. No textbook, not even the best one, and no grammar guide, not even this one, can give you that. It's all up to you.
Don't think. Feel.
Video link: Stephen Krashen on Second Language Acquisition at Pagoda Academy in Busan Part 1
Part 4: Communicating
No matter how stubborn the questgiver, rat tails will solve anything. RPG cliches tell us so. Even heroes need to make conversation. At least understand.