Examples of how to use した, しも and もと for 下?

Question

So far I learned that these three words mean "below" or "bottom", is it right? If so, are they interchangeable? Could you give me some example and preferably some explanation about how to use them?

Answer 1

下{した} is the more common of the three by a huge margin, and what you'll want to use at least 99% of the time. It's a completely basic word for "down", and has a huge variety of uses. It's the counterpart of 上{うえ}, the common word for "up".

下{しも} is a much, much rarer word, and I find it's mostly encountered in a small handful of set expressions, such as 風下{かざしも} "downwind", 川下{かわしも} "downstream" and 下{しも}ネタ "dirty jokes" (as in "jokes about the lower part of the body). It's the counterpart of 上{かみ}.

下{もと} is also restricted to certain specific uses, and is perhaps the furthest away from the basic meaning of "down". It's a variant of 本{もと} or 元{もと} meaning "origin", and generally refers to a location rather than a direction. Its uses include indicating the "root" or "base" of something (eg. 桜の木の下{もと}に "at the base of the cherry tree"), referring to a person as a location (eg. in the expression 親の下{もと}を離れる "to move home away from your parents") and referring to a higher power that one exists "under the authority" of (eg. 厳重な監視の下{もと}に置く "to place under strict surveillance"). This one doesn't have any "up" equivalent that I can think of.