What does it mean when I see a bracketed capital letter like this within a quote?
"[H]e was the one who admitted to the crime."
Is this an error - and if not, could you kindly provide a quick example?
I saw it on TV in an excerpt and had written it down last night because I was curious. That's all I have.
A court case on 'Court TV'.
When you provide quoted material, you often take just a part of it.
To make what was quoted grammatical or to help it make sense, you put something you changed into brackets to show it wasn't exactly as it was quoted.
For example, if I wrote "Bhai is a great contributor to the forum. You can always count on him for great answers." but you wanted to quote only the second part, you'd write:
According to Barb_D, "You can always count on [Bhai] for great answers."
If you fix grammar, you put it in with brackets. "Jamie and Pete is great." According to Barb_D, "Jame and Pete [are] great."
If you took the middle of a written sentence that actually had a lower-case letter, you would make the clause start with a "[Capital letter] so it looked right."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.