The words for “love” in Japanese correspond pretty well to the Greek words for “love.”
ai~agape (selfless love)
koi~eros (romantic love)
I can’t seem to find an equivalent for storge (στοργή) though. Heck, I’m not even really sure what storge means. The Wikipedia article for C.S. Lewis’s The Four Loves (I haven’t read it myself) describes it as being the love that forms between people who have bonds without any sort of choice or compatibility beforehand, e.g. the love between parents and their children. But whereas this description sounds like it also fits the relationship between e.g. siblings, the Greek dictionary I have just defines it as “love, affection, esp. of parents and children.” So I don’t know if it refers specifically to parent-child love or the more general definition on Wikipedia. Then again, I don’t know of a corresponding Japanese word in either case, so for the purposes of this post I guess it’s a moot point. For parent-child love, you would probably just use ai, or oyako-doushi no ai if you wanted to be really specific.
Whereas Greek has verb forms for each of its words for love (ἀγαπάω, ἐράω, φιλέω, and στοργέω), Japanese doesn’t, except for ai, which has a corresponding verb ai suru (which basically means “to do ai” literally). There is a word koi suru, but I think that refers more to falling in love than being in love; you might hear someone say Tanaka-san wa koi shita for “Tanaka-san fell in love,” but I don’t think you’d hear Tanaka-san wa koi shite iru for “Tanaka-san is in love.” For that, you’d just use suki, as in Tanaka-san wa Yamada-san no koto ga suki da or something like that. Suki can also be used as a general-purpose word like “like” or “love” in English, but I think Japanese people would more often express how much they like something in terms of how good the thing is rather than how they feel about it. So if you really like, I don’t know, bread or something, then rather than say “I like bread” (*Ore wa pan ga suki da), you’d say “Bread is good” (Pan tte oishii yo ne). I’m not sure if I’ve actually read that anywhere (I think I might have), but that’s the impression I get from manga and whatnot. Yuujou would be expressed in terms of nouns—you might say something like Tanaka-san wa tomodachi da for “Tanaka-san is my friend,” or if you’re really close you might use shin’yuu or yuujin (not sure what the difference is) instead of tomodachi.
Speaking of shin’yuu, the differences between the Japanese words for “friend” are also pretty interesting. The lowest rung on the friendship ladder would be kaomishiri, which just means someone whose face you know. This one is so distant that it’s rarely even used (judging by anime and manga). Above that would be shiriai, which is an acquaintance—someone you know but aren’t particularly close to. Then above that would be tomodachi, which corresponds pretty well to “friend” in English, though I’ve read that English speakers use “friend” more loosely than Japanese speakers use “tomodachi.” And above that would be shin’yuu, which would be a really close friend. My brother actually makes a perfect example of the difference between tomodachi and shin’yuu. In high school, he made one friend that really completely clicked with him. They’re still keeping in touch even after graduating from college, and they’re always fooling around together if only on the internet (their banter on Facebook is pretty funny). But my brother never got that close to anyone in college. He did have a bunch of people he would hang out with, and they do still talk and such, but he says he never felt like he could completely relax around his college friends. In Japanese terms, the one high school friend he’s really close to would be a shin’yuu, while his college friends would be tomodachi.
If koi and eros really do mean basically the same thing, then that actually makes me much more comfortable with eros as a word. I always equated eros with “lust,” probably because of the connotation of “erotic” in English, but koi sounds much more sentimental or romantic than that. Then again, I’m pretty sure the ancient Greeks had rituals in honor of eros that involved, uh, sacred prostitutes. So maybe it really does mainly mean lust…
One thing that kind of bothers me about the image of the word agape among people who study these sorts of things nowadays: agape can mean selfless love, and when used by Christians that’s generally what it refers to, but in colloquial Greek it was every bit as ambiguous as the English word “love.” Even to this day in Modern Greek, that’s how it’s used. Heck, I think I remember seeing one example sentence in an Ancient Greek dictionary where agape was applied to the kind of love a man has for his favorite food.