The most formal and neutral way to say 'I love you' in Japanese would be :
私はあなたが好きです。/ Watashi ha anata ga suki desu.
Let's decompose this :
- 私 / 'watashi' = I, me
- は / 'ha' (pronounce 'wa') is the particle for the subject.
- あなた / 'anata' = you ; which you can replace with 君 / 'kimi' or おまえ / 'omae' (more familiar forms).
- が / 'ga' = to make it simple, it's the particle for the object.
- at last, 好きです / 'suki desu' means to love, or 好きだ / 'suki da' or 好き / 'suki' in the familiar form. You can add 大 / 'dai' before 'suki' to really mean it, which gives us 大好きです / 'daisuki desu'.
The whole first part (私はあなたが / 'watashi ha anata ga') is quite formal and not required, that's why you can hear a mere 大好きだ / 'daisuki da'…
More precisely, the Japanese character (kanji) for love is the same than in Chinese : 愛 pronounced 'ai' (at least sometimes). You can create a verb from it : 愛しています / 'aish*teimasu' in the polite form, 愛してる / 'aish*teru' in the neutral form. Its use is the same than 好きです / “suki desu”, except you have to replace が / 'ga' with を / 'wo' (pronounce 'o'), giving us 私はあなたを愛しています。/ Watashi ha anata wo aish*teimasu.
愛 / 'ai' and its derivatives are more meaningful than 好き / 'suki'. You can 'suki' baseball or ice creams, but you can only 'ai' someone.
