請問比較輕的英文是怎樣寫?

An orange is lighter than watermelon - that's right, but you can say the other way around - Watermelon is heavier than orange, "an" is not applicable at all as we all know oranges are generally lighter than watermelon.

An orange is lighter than a watermelon.

不是，是 An orange is light than watermelon.
希望幫到你！

橙比西瓜輕
An orange is lighter than watermelon

LIGHTER要連THAN用先係比較輕，只用LIGHTER係名詞既打火機 。