Why is there no с before the ручкой?
It depends on the situation. If you are using something as an instrument, you don't put с as per the answer.
If the subject is actually -with- something, you use с. For example, с гривами is "with mushrooms."
GarettTree - you have a typo - it's "с гриБами" ^_^
What is the difference between ркчка and перо , or are they interchangable?
"перо́" is a feather and also the colloquial name for a fountain pen (перьева́я ручка) and a stylus for a digital graphic tablet.
How do you say, "I am drawing a pen?"
Я рисую ручку.
Does the use of the instrumental case make it "a pen" (in general) instead of "the pen" because "the pen" was marked incorrect.
"The pen" should be accepted. It is a correct answer.
In English people often say "I am drawing in pen," as well.
Talk about самоуверенность!
Why is it not : I draw with a pen. Why is it present continuous?
It can be either. Both are accepted.
'I am drawing with my hand' is equally valid, no?
That would be «я рисую рукой».
Yes, but ручка is a diminutive form of рука. Still, formal language should be used, at least with Duo on Duolingo, and it is very unlikely that you would draw only with your arm/hand.
Unless you are a very small child, you won't say "я рисую ручкой" when you mean your hand, and not a pen.
I see what you did there. The -ой ending makes it instrumental.
But does adding the <с> change the meaning?
Yes. "С" is used when you want to say "together with" rather than "by means of". Я рисую ручкой - I am drawing, using a pen for that purpose. Я рисую с ручкой - a sentient pen and I are drawing together.
Pens come with artificial intelligence nowadays ... :-D
Well, now you're prepared to describe that event in Russian :-D
I'm not keen on this one, now I am confusing the two in other exercises! :-/
Pronounciation: So you don't hear/pronounce the "o" ручкой at all, do you?