Why are military ranks so hard to spell?
Sergeant - ?
Ensign - ?
Lieutenant - ?
Captain - Only easy because it’s so common in other contexts.
Colonel - huh?
Private
Admiral
General
Commander
Some are difficult. Some aren’t.
The spelling of most of them comes from the history of the language.
“sergeant” is from the middle english word for “servant” which was “segent” or “serjent.” From the Latin “servir = to serve.”
“ensign” is from the Latin “insignia = insignia, flags.” Why the rank is called this is another question that you can look up.
“lieutenant” is from the French words for “place holding.” It originally designated an official who was authorized to act for ( in lieu of which is the same thing as in place of) another.
“colonel” is from the Latin “colonna = column via old Italian collonello for a column of soldiers.” A Colonel commaned a column of soldiers.
English spelling is the pits and the examples you cite are relatively benign for cumbersome spelling.
Incidently, I was an Army Lieutenant for three years and always abbreviated it Lt. because I couldn’t remember for sure how to spell it.
Consider the unfortunate British Army lieutenants, their rank is pronounced left-tenants
Perhaps off-topic, but how did the rank “private” get its name?
Brigadier General = 1 star
Major General = 2 stars
Lieutenant General = 3 stars
What’s funny is that a major outranks a lieutenant but in the general ranks it’s reverse. And brigadier (from the sound of the word) sounds like it should outrank both.
I’d like to hear the story behind (heh) the Rear Admiral rank. It sounds like it’d be a funny story but I’m sure it isn’t.
Well, it makes sense if you realize that “Lieutenant” in “Lieutenant General” and “Lieutenant Commander” means “assistant.” (Or, in lieu of.) Like a Lieutenant Governor.
In the US Army, a regular General has four stars, so it makes sense that his Lieutenant has three.
A Brigadier General traditionally led a Brigade.
Major General was originaly called Sergent Major General, after a time the Sergent got dropped, leaving a Major General lower then a Lieutenant General.
Back in the days of fighting sail, the ships formed a line when they apporched the enemy. The Rear Admiral was incharge of the last ship in the line. After the line would pass the enamy, they would turn around so that the last ship was now in front of the line, under the command of the Rear Admiral for that pass.
some of the stuff that is put on instead of commercials on armed forces network overseas. I’d rather have the commercials, they at least change over time.
Each example you cite is a word derived from, and spelled according to the rules of, a non-English language. This does not seem to support your claim that “English spelling is the pits” (emphasis added). If anything, it would seem to justify a claim that the spelling of non-English languages are the pits. 
The spelling of English is so confused because English is more of a creole than a real language. It is derived from at least five languages (German, Norse, French, Latin, and Greek (Greek is used mostly in technical terms, but there are a lot of them) with imports from over a hundred others.
Add to that the changes in pronunciation over the centuries (the “gh” in the word “right” reflects the pronounciation prevelant at the time that spelling was standardized), and what is left is a huge mishmash.
The odd spellings reflect and preserve the history of the language. I wouldn’t have it any other way.
Me either, but English spelling is still the pits.
As for “private”, IIRC in the period of the late Middle Ages/Renaissance, when all the officers were still some form or another of nobility/gentry but you already had units where the ranks were made up of paid free commoners, the folks on the line would be hired as “private” individuals.
The pronunciation thing with “Colonel” as “curnel” is a conflation of the above mentioned “colonnello” (leader of a column) with the Spanish “Coronel”. (dunno if the Spanish came up with it on their own: corona–>coronel, Officer of the Crown?)
Another way of remembering the MajGen/LieutGen sequence is to remember that in Central European armies the upper rank is “Colonel General”, thus replicating the sequence in the field grades (Major, LtCol, Col)
As for the “leftenant”, heck, maybe someone saw it written (in old type) as “LIEVTENANT” and it stuck? ( I AM making this one up…)