All right, everypony, gather 'round! Jot's going to run a little class here.

There's a mistake I see a lot in FiM fanfiction that I'd like to address. This is probably a futile gesture, since I suspect everyone who reads this knows what they're doing and the bulk of the errors come from our non-English-speaking friends, but... I dunno, maybe you can link people to it or something. If you're a grammar nazi like me, anyway.

Okay, so. Can you spot what's wrong with this sentence?

"I want every pegasi in the air five minutes ago! Move!"

That's right, "pegasi" is plural, and it doesn't fit here. I should have used "pegasus" instead. I see this a lot with people who I wouldn't think would ordinarily make mistakes like that, so I have to assume the -i ending is throwing people. It works just like any other plural, folks.

But you're talking about a group of pegasi, right? Shouldn't you use the plural form?

Well, sort of. Some phrases, like "every", turn it into a singular because you're applying the rest of the sentence to each individual member of the group. It would have been correct if I said,

"I want all pegasi in the air five minutes ago! Move!"

Other phrases pick out some specific member, like "the last". For example,

The last pegasus to leave turned out the lights.

You do have to be careful with some phrases, though, because they can go either way depending on how you use them. For example, compare:

The first pegasi through the classroom door stopped in shock.

to

The first pegasus through the classroom door stopped in shock.

Both can be correct, depending on whether you're talking about the first group of pegasi to enter the room, or only the very first student. If you're not sure, just test it with another plural word -- say, "ponies" -- in place of "pegasi". To go back to my first example,

"I want every ponies in the air five minutes ago! Move!"

Obviously wrong, right? Right.

Well, that's pretty much all I have to say on that subject. Class dismissed. Now go do it right, or next time I'll tell you why you're wrong about the Oxford comma.

Edit: Oh yeah, and here's one I forgot: if you're using "pegasus" as a modifier to another noun, it's always singular and you pluralize the "main" noun as necessary.

Celestia's pegasus guards bowed.

Again, it would be clearly wrong to say, Celestia's unicorns guards bowed, so "pegasi" would be wrong as well.