The structure and rank categories of the military can be a little confusing. The military consist of personnel with names and numbers that signify who answers to who. There may be minor variations in different branches of the military, but military personnel must use certain forms of address when speaking to each other. Even if you are not in the military, you can show respect to the men and women of the military who put their lives on the line to protect this country by addressing them with the correct titles.
Someone of junior rank may address a senior officer by his or her rank and last name, or "Sir" or "Ma'am," but they are not to use the senior officer's first name.
Lieutenant generals, brigadier generals, and major generals are all addressed as "General."
All lieutenants are addressed as "Lieutenant." Only when writing is "First" or "Second" used.
A master sergeant is called, "Master Sergeant" and their last name or "Sergeant" and last name.
A chief master sergeant is called "Chief" plus the last name -- never "Sergeant" only.
Senior officers may call or speak to a junior officer by their last name only, but the junior officer can not do the same for a senior officer.
Regardless of the military's official title, chaplains are addressed "Chaplain."
Sergeants major are addressed as "Sergeant Major."
First sergeants are addressed as "First Sergeant." Other sergeants are addressed as "Sergeant," and a corporal is addressed as "Corporal."
Privates are generally orally addressed by only their last name, but "Private" and their last name is usually used in written communication, and special situations like standing before a panel or board such as the UCMJ (Uniform Code of Military Justice).