Unknown Command?

First, I am not a regular AutoCAD user. I regularly use another brand 3D
modeling software. Basically, we only have a seat of AutoCAD (it is
actually Mechanical Desktop 5 with ACAD 2000i included) to view customer
drawings and do minor revisions. I have never had any formal training in
AutoCAD. So please bare with my lack of intelligence.
On my first computer that I had when I started working here, I taught myself
ACAD enough to where I could do basic things. I have since gotten a new
computer and installed MD on it and set it up just as it was on my old
computer. I have noticed that for some reason, there are several commands
that give me an error of "Unknown command - Press F1 for help". I have used
ALL of these commands when I used ACAD on my old computer. Some of these
commands are:
properties
find
dwgprops
3dorbit
adcenter
etc.
Also with most (if not all) of my snaps which include:
tt
from
endp
mid
int
appint
cen
qua
tan
per
par
etc.
And also when I try to go to my:
snap settings
grid settings
ortho settings
polar settings
osnap settings
otrack settings
but oddly enough I CAN get to my line weight settings
Does anyone know what might be going on here? If so, could you please
explain the fix in simple terms. Remember I am not very ACAD knowledgeable.
Thank you,