Not all the time but when I start the car it starts very hard. When it starts hard like that and I wait a few second and put it into gear it slams into gear and sometimes won't move.

Get it serviced?
hard starts are usually never getting a tuneup, (like spark plugs?)
slamming in to gear is normal if RPM is too high in park, is it.
wont move , in any gear, car wont move even in reverse.
and RPM of engine does what , sticks low, like 800 rpm
or screams at the advancement of right foot.
need lots of details. we cant drive car, but if could in 5min flat all that is clear,
eg:
transmission slips
engine weak
engine racing in park too high.
wont start and misfires when it does.
is fuel fresh, not year old.
ever get a new air filter and fuel filter. if filter is outside tank.