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- [Voiceover] Hello, Grammarians. We are once again learning how to master time and become Time Wizards, which is, of course, what you will be if you master all the tenses of English. But, if you want to become an additional Time Wizard, if you want to get, I don't know, a second hat, because everyone knows all wizards wear multiple, simultaneous hats. If you want to wear a second hat on top of your other Time Wizard hat, that's the silliest thing I've ever said, then you will have to learn the Prepositions of Time. Now here's something weird and cool about Prepositions of Time is that once upon a time all of these were physical prepositions. Like "Before" and "After" just used to mean behind and in front of. They later took on this additional connotation of time just over the course of English-Speaking history. People took this literal meaning of "Before" and "After" and made it representational. This moment occurred in front of this one in time, but behind this other one in time. It's a metaphor. We're using space to represent time. Anyway, I just thought that was really cool. Let's talk about how all of these work. I'm just going to list a couple of the most famous Prepositions of Time and write an example sentence. So let's go through these. "After" and "Before," as we've established, these are time relationships that refer to something happening after. So when something is completed, say, "The bats come out after the sun goes down." And before this occurs prior to some point in time, so it is behind an action. So you can say, "Can you take out the garbage "before you leave the house?" "At" is very precise. When we're talking about "at" we're talking about a single moment in time. We could say, "The vampire wakes at 10 p.m." There he is emerging from his coffin. There's the, oh that's not the sun. The sun would burn a vampire. No that's a little clock. It's got a tail because it's like a little wacky cat clock. No, Bat Clock. That doesn't have a tail. It's got a little stubby tail. Doop. Ah, I really want a Bat Clock now. Anyway, okay. "By," this is a really precise end time, but not a very precise beginning time. So you could say something like, "This place had better be clean by 3 p.m., buddy." If you say something like that, you're not especially concerned that the place might be clean before three. That would be nice, but it's only relevant to you that the cutoff time is at three o'clock. So the end is precise. That's the connotation there. But the beginning is not. "For" denotes duration. How long something has been going on. So you could say, "I've been a chef for 40 years." I haven't, but that would be cool and difficult. But you know what, this is Khan Academy, you can cook anything. "In" denotes a bounded duration. So it's something that lasts for a specific amount of time, like a limited period. Okay, so let's just say bounded duration. So that covers usage like, "In March" or "In the Middle Ages." Both of those things are like set periods. March has a beginning and an end. The Middle Ages have a beginning and an end. It's a bounded duration. "On" has a specific connotation. It's something that happens on a specific day. You could say something like, "On the 4th of July, "many Americans watch fireworks and eat encaged meats." Mind you not everybody eats hot dogs or likes fireworks, so I said many not all. "Since" is kind of like "By" except it's more about the precision of the start point rather than the end point. So, precise beginning. "Since 1974, our company has made "nothing but toasters." "Until" is also precise, but it's a precise ending time. "You have until midnight to rescue the Ambassador, break the curse and save Prince Wilbur. All right. So there's a precise ending there. You have "until midnight," and then you can't rescue the Ambassador, break the curse or save any princes. But what you can do is learn anything. These are some of the most essential Time Prepositions. David out.