It's not "an UFO" it's "a UFO". Even though, yes, the word UFO starts with a vowel, when you pronounce the letter "U" - the sound starts with a "Ɉ" sound ("ya") which is a palatal consonant, meaning you'd use an "a" instead of an "an".

It works the other way, too. You don't say "a hour" you say "an hour". Even though an "H" is a consonant, it has a vowel sound in the word "hour" so you'd use an "an".

Hopefully, if the UFOs ever *do* come to Earth, you'll be able to speak and write English more legibly and an interstellar incident won't happen because you've used the wrong article.

Grammar is important. It *IS*!

I highly doubt Grammer is anything close to being important.And if aliens come, they would probably kill us before we even said a word, So stop being such a Grammer Nazi.