okay i'll do my best

a semi-colon does not separate two different ideas; it means there's a connection between them, either in the sense of explaining what has come before it or expanding on it in some sense. (it's basically the same as the difference between a period and a comma, which makes sense considering the punctuation marks themselves.) a colon is used when you're about to list something -- you "listed" a comical but meaningful explanation of what grammar means, and yes, it's still listing even if there's only one thing.

sometimes a semi-colon and a colon can be used almost interchangeably, for instance when what's after the colon explains what's before the colon: in other words, like this sentence. compare that with this sentence; in the first sentence, what came after the colon directly explained the content before it, whereas in this sentence it expands on the content before it, explaining WHY you should compare the last sentence to this one.

that was pretty meta.

that's not entirely an adequate explanation, but it's the best i can manage. my understanding of punctuation is by and large intuitive, so i am pretty shitty at explaining anything that requires any nuance or technical insight. i just know from reading what is right and when.

as for the capitalisation, well, you didn't start a new sentence, and the "the" is not anyone's or anything's title, so it doesn't need to be capitalised. over-capitalising is an easy temptation to fall into.