I suppose this is not a repeated question.
While it's natural to capitalize the first letter in a quoted sentence (even it's split into two parts), I want to know should I capitalize the first letter of quoted vocabulary (such as a noun, or something more special) which shows up at the first place. e.g.
(Each of the following is a complete sentence.)
(Of course, there're plenty of alternative expressions.)
"red" is a concept of color.
or
"Red" is a concept of color.
More specially, what about
"a2b7x" is the code I got.
(Capitalizing "a" would just give a wrong code.)
and
The sentence started without capitalization, reads as "i start with a lowercase 'i'".
The punctuations and the capitalization rules are there to serve the purpose to carry information explicitly and accurately. So I think that forms are less important than content/meaning and we may allow the existence of sentences such as
"a" is a lowercase letter.
It's really strange to say
"A" is a lowercase letter.