Answer by Professional Japanese Teacher
I love you
The Japanese word for “love” is 愛(あい) ai and it generally refers to romantic love. It usually appears in this phrase 愛(あい)してる aishiteru (I love you).
However, this phrase is rarely used in daily life as it is considered too direct. It is said that Japanese people prefer indirect ways of expressing their feelings and that it is common for couples to show love for one another instead of stating it.
If you were to use this phrase on someone, it would mean that you are engaged to the other person and are seriously considering marriage and spending the rest of your life with them.
The concept of “love” is different in Western and Japanese culture. While it is common to say “I love donuts!”, “I love Italian food!” in English, it would sound strange if you were to go around saying “ドーナツを愛している Doonatsu o aishiteru“ or “イタリア料理(りょうり)を愛してる Itaria ryoori o aishiteru” in the same manner. In this case, 好(す)き suki (like) is used when talking about things you like, for example, “ドーナツが好(す)きです Doonatsu ga suki desu (I like donuts)” or “イタリア料理(りょうり)が好(す)きです Itaria ryoori ga suki desu (I like Italian food)” You can also use suki for people, for example, 星野源(ほしのげん)が好(す)きです Hoshino Gen ga suki desu (I like Hoshino Gen).
However, when you use suki to express feelings for someone in a confession of love (告白(こくはく) kokuhaku), the meaning of “suki” can change to “love”. A confession may sound like this: 吉田(よしだ)さんのことが好(す)きです Yoshida-san no koto ga suki desu (I love you, Yoshida). こと koto means “things”, so by adding koto to a person’s name, it means you like/love everything about this person.
So to summarize, use suki if you like things or someone in general (not in the romantic way). Suki becomes “love” if you are confessing to someone or if you are already in a relationship with someone. Only use aishiteru if you seriously love that person.