One of the most popular phrases in any language is probably "I love you." In Japanese, "love" is "ai （愛）," and the verb form "to love" is "aisuru（愛する）." "I love you" can be literally translated as "aishite imasu （愛しています）". "Aishiteru （愛してる）," "aishiteru yo（愛してるよ）" or "aishiteru wa (愛してるわ,)" is normally used in conversation.However, the Japanese don't say "I love you" as often as Western people do, because of cultural differences. I am not surprised if some Japanese say that they have never used these expressions in their life. The Japanese generally don't express their love openly. They believe that love can be expressed by manners. When they put their feelings into words, it is preferred to use the phrase "suki desu （好きです）". It literally means, "to like." "Suki da （好きだ）," "suki dayo" (好きだよ, male speech) or "suki yo (好きよ, female speech)" are more colloquial expressions. There are many variations of this phrase, including regional dialects (hogen). "Suki yanen （好きやねん）" is one of the versions in Kansai-ben (the Kansai dialect). Since the phrase can also mean "I love it," and because of the popularity of Kansai-ben, it is used as the name of an instant noodle soup product. If you like somebody or something very much, "dai (literally means, big)" can be added as the prefix, and say "daisuki desu （大好きです）."

There is another word to describe "love" which is "koi（恋）." The kanji character for "kokoro (心, heart)" is included as part of both kanji characters. Compare the kanji characters below (From left "kokoro," "ai" and "koi").

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Both "ai（愛）" and "koi（恋）" can roughly be translated as "love" in English. However, they have a slightly different nuance.

Here are some words including "ai（愛）" or "koi（恋）."
 

"Koi" is a love for the opposite sex, or a longing feeling for a specific person. It can be described as "romantic love" or "passionate love." While "ai" has the same meaning as "koi," it also has a definition of a general feeling of love. "Koi" can be selfish, but "ai" is a real love. Here are some lines that explain them well:

Koi is always wanting. Ai is always giving.

"Renai（恋愛）" is written with the kanji characters of both "koi" and "ai." This word means, "romantic love." "Renai-kekkon （恋愛結婚）" is a "love marriage," which is the opposite of "miai-kekkon (見合い結婚, arranged marriage)." "Renai-shousetsu （恋愛小説）" is "a love story" or "a romance novel." The title of the movie, "As Good As It Gets" was translated as "Renai-shousetuska (恋愛小説家, A Romance Novel Writer)."

Here are some proverbs that include "koi."
 

"Soushi-souai（相思相愛）" is one of the yoji-jukugo（四字熟語）. It means, "to be in love with one another."

The Japanese sometimes use the English word "love" as well, though it is pronounced as "rabu（ラブ）" (since there is no "L" or "V" sound in Japanese). "A love letter" is usually called "rabu retaa（ラブレター）." "Koibumi（恋文）" sounds a bit dated, and it is more a literary word than a conversational word. "Rabu shiin（ラブシーン）" is "a love scene". Young people say "rabu rabu (ラブラブ, love love)" when they are very much in love.

"Ai（愛）" can be used as a female name. Japan's new royal baby was named Princess Aiko, which is written with the kanji characters for "love（愛）" and "child（子）."However, "koi（恋）" is rarely used as a name.

There is another word pronounced the same as "ai" and "koi". Since their meanings are distinctly different, I don't think there is any confusion between them when used in the proper context. With different kanji characters, "ai（藍）" means, "indigo blue," and "koi（鯉）" means, "carp."　Carp streamers that are decorated on Children's Day (May 5th) are called "koi-nobori（鯉のぼり）."