What is the general syntax of a Unix shell command?

Question

In particular, why is that sometimes the options to some commands are preceded by a + sign and sometimes by a - sign?

for example:

Answer 1

It's left to apps to parse options hence the inconsistency. Expanding on your sort example these are all equivalent for coreutils:

sort -k3
sort --k 3
sort --key 3
sort --key=3
_POSIX2_VERSION=199209 sort +2