Grammar is very important in communications.  It is the tool by which our 
attempts to convey meaning are made clear and (hopefully) concise.  If your

grammar is not correct, the meanings of your writings will not be clear. 
Since 
we are a text-based MUD, writing is essential as it is the only way by
which 
we can express ourselves in-game.

For example:
Who is a subjective pronoun, meaning that it is the subject in a 
sentence or phrase.
"Who goes there?"
"Who was that?!?"
"Who left this in my castle?"
Whom, on the other hand, is an objective pronoun.  This means that this 
word is the object of a particular phrase or sentence.
"That girl is a guest of whom?"
"She needs to look after whom?"

In this case, the distinction is subtle; most people will correctly assume
the
original meaning if the incorrect word is used.  However, the reader is not

always so lucky.

Some reference help files:
possessives
it's, its
you're, your, ur
who's, whose
scared, scarred
they're, their, there
two, too, to
no one, noone
you, u
numbers