uncertainty about command "test"

dear gnu-people,

trying to learn programming the bash-shell (of the valhalla release by
red hat linux) i have encountered many puzzling problems that eventually
i could find the simple solution to. many times this was due to some
detail i had overseen.
but now i have a problem that i cannot explain to myself:

the command "test" seems not to work as it should.

i want to test an empty file "xyz" with the following syntax:     test
-s xyz && echo empty
this doesn't work. (command not found) if i spell the same command as: [
xyz -s ] && echo empty
i get the remark "[: unary operator expected" but if i type: [ xyz-s]...
or  [ -sxyz ]... or  test -sxyz...
the command seems to work. it seems unlogical to me that the blank is
not wanted by the command but i could put up with that. but now: if i
run the same test with a not empty file it it still found empty by the
command (it exits with status 0). what gives me to think a lot that if i
enquire the exit status with $? i get this remark "bash: 0: command not
found".

i hope you understand what i'm talking about and have the time to clear
me up about what i'm doing wrong.

best regards from jan-herbert damm, berlin, germany