it's because Gordon is a scientist and could muddle his way through the day not speaking but Shepard is a soldier, a lack of communication on the battlefield could lead to unnecessary casualties.

Cause Shepard's a Marine. He needs to talk to give orders and answer radio calls.

because they don't now if adrian is just being quiet or mute

Only three things to say:

1. Interesting idea, but the execution seems to be a little awkward.

2. To answer your question, various Valve employees have stated that Gordon Freeman was intentionally made mute in game to enhance player's immersion and experience, while whether he's actually mute or refuses to talk for some reason is a topic of countless debates over the internet. As for Adrian Shephard, however, it's heavily implied in game that he actually speaks.

3. Read the rules. Author's notes as chapters were not allowed, and definitelly not in a "pop-quiz" form (anything interactive, I think). In any case, you could have slipped that question as the last paragraph on the previous chapter, not create a new one.

Simply because Gordon has been noted by other characters to not speak, or at the very least be very, very quiet.

WIN

Even though I haven't played half life yet (I only have the 2nd one) and I don't know who this other dude with the casserole is, this made me LOL!

:)

hehehehehehehehehe.