Answer - Fly v. Flies in American/British English

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snycock asked: What are the US and UK differences between 'fly' and 'flies'?

With examples from NCIS and Some Mothers Do ‘Ave ‘Em.

It's always interesting comparing word usage in different countries which speak the same language, and that usage can often be markedly different when looking at the way a particular word is used in the UK and the US. But in this instance, there actually aren't huge differences in the majority of the definitions for both the verb and noun forms of "fly" -- at least as outlined in the Oxford Dictionary. One difference is that American English does have the concept of the "fly out", a baseball term completely unused in the UK:

Ortiz flied out to left field.

Additionally, the definition of "fly" as successful is considered an American informal usage:

Tony volunteered to follow the suspect to Hawaii, but the idea simply didn't fly with Gibbs.

Moving to plurals, the noun "the flies" refers to "the space over the stage in a theatre", and you'll note that it must always be used with "the" as part of the noun. Also, the plural of a one horse carriage, called a "fly", is usually "flys". This leaves the only noted difference in the dictionary associated with the meaning "an opening at the crotch of a pair of pants, closed with a zipper or buttons and typically covered with a flap", the difference being that "fly" is the standard usage in American English and "flies" a frequent usage in British English. That's what I'm going to focus on for this answer.

According to the Online Etymology Dictionary, the first usage of "fly" in reference to some sort of flap or cover was in 1810, where the notion of "flapping as a wing does" led to the noun sense of "fly" meaning "tent flap". Interestingly, if you look at a modern tent, the "fly" isn't necessarily something that covers the opening or entrance, but is usually an outer layer which is suspended above the tent itself to keep condensation or sun off the tent's occupants. Essentially it's a tent without walls, and it can be used in conjunction with the tent, or in a stand-alone manner. Coming back from that tangent, by the mid-1840s, "fly" had also come to mean the "covering for buttons that close up a garment". Since then, rather than simply referring to the flap of cloth that covers the buttons, or the zipper, which close up a garment, "fly" now refers to the whole set of fastenings as well as the piece of fabric which covers them.

One note here is that, while "fly" always refers to the opening at the crotch level of a pair of pants or trousers, it is possible to use "fly" as a modifier in combination. An example of this would be when talking about a "fly-fronted shirt", where the shirt would have a piece of cloth covering up the buttons -- though the Oxford dictionary only references this usage in the American English definition of "fly".

Let's see some of this in action.

"Tony, your bug is down," Ziva said, obviously uncertain if she was using the expression correctly.
"What?" Tony responded, for once unable to provide his usual correction to the Israeli agent's mangled English.
"I think she means your fly is down," McGee stated, smirking as Tony quickly remedied the situation before anyone else noticed.

A similar example in British English would be:

"Frank? Your flies are open."
"Ooh, Betty," Frank exclaimed, struggling to pull up his zipper and jamming it on his underpants in the process.

Even though this example has Betty talking about "flies", she could also have said to Frank that "your fly is open", because both "fly" and "flies" are accepted in British English. Also note in this example that the reference to "flies" is plural, so we need to say they "are open", not "is open".

Here's an example of using "flies" incorrectly:

Frank tried to fumble both his flies open as quickly as he could.

Now, knowing Frank, it's entirely possible that he was wearing two sets of trousers, both of which had "flies", but let's assume that isn't the case. So even though "flies" is plural, we're still referring to a single set of fastenings, so we should take the word "both" out to be correct:

Frank tried to fumble his flies open as quickly as he could.

This answer wouldn't be complete without discussing the actual articles of clothing in which a "fly" would exist -- pants, slacks, and trousers. In British English, the "outer garment covering the body from the waist to the ankles, with a separate part for each leg" are referred to as "trousers", while in American English these are typically called "pants", and sometimes "slacks" -- though "slacks" can also refer to more casual trousers in the UK. To make things a little more confusing, "pants" are generally an abbreviation for "underpants" in British English, so please don't have your character struggling with their "pants" if you're writing something using British English, as they may be in more of an undressed state than you think!

That pretty much covers it. Hopefully in the future you won't find yourself "flying at half-mast", doing any "low flying", leaving the "barn door open", needing "Hal to close the pod bay doors", having "Ensign Hanes reporting a hull breach on the lower deck", paging "Mr Johnson", needing to "bring your tray table to an upright and locked position", or having a "security breach at Los Pantalones" when it comes to using "fly" and "flies" in the future.


Sources
http://oxforddictionaries.com/definition/english/fly?q=fly
http://oxforddictionaries.com/us/definition/american_english/fly?q=fly
http://www.etymonline.com/index.php?term=fly
http://en.wikipedia.org/wiki/Fly_(tent)