まで means "until" or "up to" and are used to describe ongoing actions up until a certain time. までに means "by" and are used for actions that are completed before a certain time. The difference is that まで describes continuous actions while までに does not. I think of it like までに describes a deadline while まで describes a continuous action up until that time.
It's not, you're far from being the only one that got confused because of that.
Conceptually, まで just mark a "limit" (physical limit, temporal limit, etc) or the extent of something.
六時までやりましょう -> Let's do it until 6 一緒に駅まで行きましょう -> Let's go until the station together
That's for the most common examples. But まで can be used for case a little bit different too.
家から駅まで、道の両側に木があります。-> From my home to the station, there are tree standing on both side of the road.
And in that case, it's not an "ongoing action". It can also be used for more abstract things.
上手になるまで勉強します -> I will study until I'm good
Now for までに, the deadline idea is really a good example and useful way to remember it (except for the part that it's limited to temporal case only). に kinda give the idea of "target". It's still a limit but also a place/time/state by which something happen before you reach that place/time/state.
Okay, my brain is fried now trying to find a good explanation. I will just give you a few example
六時までに帰ってください。-> Please come home by 6 学校へ行くまでに雨が止んだ。 -> The rain stopped before I got to school 怒りが爆発するまでに話し合おう。 -> Let’s talk it out before I explode with anger.
Hope those will help. And sorry to give up mid way x) But if it doesn't, don't worry too much. I got so annoyed with the まで/までに pair that I just stop studying it for a long time. But after immersing for some time (and also getting a better grasp of まで and に separetly), it became more obvious to me. So don't worry too much if you're still struggling, that's perfectly normal. And it can be better to just give up temporarily and let it mature through immersion.
