请注意该句
Thus, we need not only to build a strong body but also keep a good mood in order to face the pressures or challenges of modern society.
之中,not only...but also结构其前面加to,是需要构成固定搭配need to(即有“需要(做什么)之意”);而其后面根据语法要求实际上也应加to,只不过是省略了。
希望能够帮到您!
追答十分感谢(❁´ω`❁)