comma before direct speech/quotation: Mr.Kershner said, "You should

Well, I keep pestering you with commas.
Another usage of it is to

set off quoted material that is the grammatical object of an active verb of speaking or writing. Mr. Kershner says, You should know how to use a comma.
(wikipedia)

Let me linger for a while over these active verbs of speaking or writing.
Are they merely verbs such as write, note down, whisper, jot, mutter, splutter etc?

Thanx

I refer you again to Larry Trask :)
The section on quotations is HERE.

Now notice something else which is very important: a quotation is set off by quotation marks and nothing else. A sentence containing a quotation is punctuated exactly like any other sentence apart from the addition of the quotation marks. You should not insert additional punctuation marks into the sentence merely to warn the reader that a quotation is coming up: that's what the quotation marks are for.

Click to expand...

As with many issues relating to punctuation, I am sure there are other views.

Looking specifically at the example you give, I would not use either a comma or a capital letter. This is not an exact quotation.
Mr Kershner says you should know how to use a comma.
Change it slightly:
Mr Kershner said "You should know how to use a comma."

Hmmm, yes. Mr.Trask then goes on to explain when you should use a comma for quotations, e.g.

"The only emperor", writes Wallace Stevens, "is the emperor of ice cream." The commas here are bracketing commas, used as usual to set off weak interruptions; their presence has nothing to do with the presence of a quotation, which is itself properly marked off by the quotation marks.

I suspect I'm a bit old-fashioned about some kinds of punctuation ~ personally I'd probably always use one to introduce actual speech:

Mr.Kershner said, "You should know how to punctuate by now."