Complex sentence: It is one grade above the rank of Second Lieutenant (O-1), one grade below a Captain (O-3) and promoted after 18 (Army) and 24 months (US Air Force and Marine Corps) of commissioned service.
Split into simpler sentences: It is one grade above the rank of second lieutenant (O-1) and one grade below a captain (O-3). Second lieutenants are usually promoted to first lieutenant after 18 (Army) or 24 (Air Force and Marine Corps) months of commissioned service.