One of the many fun things about studying the history of the English language is that it explains things that can seem like mysteries.
For example, at some point you probably learned that words like "wife" and "life" are spelled with a "v" instead of "f" in plural form. Easy enough, until you found out other words like "roof" and "sheriff" don't follow this rule.
So why does "f" just mysteriously change to "v" in certain words? To solve this one, we need to look back on Old English. We also need to make some sounds. Trust us, this will be fun.
Let's start with the "f" sound. Put your teeth on your bottom lip and blow. That "fffff" sound you hear is called a voiceless sound, because your vocal cords don't vibrate when you make it.
The "v" sound is exactly the same as the "f" sound, except it's a voiced sound, one that makes your vocal cords vibrate. You can actually feel them vibrate by putting your fingers on the front of your neck and making the "v" sound. Now make the "f" sound again. Nothing, right?
Did you notice how you didn't even have to change the position of your mouth when you made these sounds? The only real difference here is that "v" is voiced and "f" isn't. This is a big clue to our mystery.
In Old English "v" was a variant of "f" that occurred in certain contexts. Old English speakers would've heard these as the same sound. That may seem strange, but there are examples of this in today's English. To help us understand, we get to make more sounds.
Try this: Hold your hand in front of your mouth and say the words "pot" and "spot." You should be able to feel a puff of air come out of your mouth when you say "pot" but not when you say "spot." That's because in "pot" the "p" is aspirated, but the "p" in spot is unaspirated.
These words contain two different pronunciations of the same letter, "p." However, as English speakers, we tend to think that it's all the same sound.
Similarly, in Old English "f" and "v" would've been two variants of what Old English speakers would've thought of as one sound, and it was absolutely predictable when an "f" would become a "v." An "f" would become a "v" between two other voiced sounds. It helps to know that in English, all vowels are voiced.
In Old English, a word like "wif," which meant "woman," was made plural by adding "as." So "wif" became "wifas," and the "f" sound became a "v" sound, because the "f" was now surrounded by vowels.
There's a whole set of words that followed this pattern in Old English -- wolf/wolves, thief/thieves, leaf/leaves, etc. However, a modern borrowed word like "gaff" is made plural by adding "s" to the end, because it's not an Old English word.
There, mystery solved. That's one down, countless more to go.
