It’s one thing to imagine something. It’s another believe it, and yet another to know it. The first step is relatively straightforward: you go from merely picturing something to yourself to thinking that’s “how it is”. I can imagine a beer in my fridge. But it’s much nicer to think there’s a beer in my fridge. Ideally, of course, I would know it. How would I do that?
Well, first of all, while I can easily believe there’s a beer in my fridge without there actually being one there, in order to know it, there has to actually be one in the fridge. My belief, we might say, has to be true. But I also have to have a good reason to believe it. Merely very strong wishful thinking doesn’t count as knowledge, even when it happens to be true. And no amount of good reasons will do if what I’m thinking about isn’t actually the case.
Those three different mental operations are useful to distinguish: imagining, believing, knowing. They are all modes of thought, I suppose. But they are distinct modes. Each can be done well or not so well. I encourage you to practice all three.