1 Say "wǒ ài nǐ" in Mandarin or Standard Chinese. This is the most common, standard way of telling someone “I love you” in Chinese.
Standard Chinese and Mandarin are, essentially, one in the same. Mandarin has more native speakers than any other Chinese dialect, and it is spoken across the majority of norther and southwestern China.
In traditional Chinese characters, this expression would be written as, 我爱你。
This expression is roughly pronounced, whoa-eye-knee". The "wo" should be pronounced in the 3rd tone (slightly dipping and then rising to a higher pitch). The "ai" is a fourth tone (sharp, falling pitch) and the "ni" is again a 3rd tone.
2 Use "ngóh oi néih" when speaking Cantonese. If speaking or writing to someone who speaks Cantonese, this would be the best way to tell him or her “I love you.”
Cantonese is another common dialect, but it is mostly spoken in southern China. Many people speak this Chinese dialect in Hong Kong and Macau, as well.
In traditional Chinese characters, this expression would be written as, 我愛你。
This expression is roughly pronounced, na(wh) OI nay.
3 State "ngai oi ngi" in Hakka Chinese. For speakers of the Hakka dialect, you should use this phrase to say “I love you” instead of using the Standard Chinese phrase.
Hakka Chinese is only spoken by a small amount of people, usually ethnic Hakka people who live in provincial areas of China, including Hunan, Fujian, Sichuan, Guangxi, Jianxi, and Guangdong. It is also spoken in various parts of Hong Kong and Taiwan.
In traditional Chinese characters, this expression would be written as,