* 下 (した): This is the most common way to say "under" in Japanese. It can be used in a variety of contexts, such as "under the table" (テーブルの下) or "under the sea" (海の底).
* 低い (ひくい): This word means "low" or "short" and can also be used to mean "under". For example, you could say "the water level is under the bridge" (水位は橋の下だ).
* 下に (したに): This phrase means "to the bottom" or "to the lower part". It can be used in a variety of contexts, such as "go to the bottom of the hill" (坂の下まで行く) or "put it on the lower shelf" (下の棚に入れる).
* 未満 (みまん): This word means "under" or "less than". It is often used in mathematical expressions, such as "under 18 years old" (18歳未満).
These are just a few of the ways to say "under" in Japanese. The best translation will depend on the specific context in which you are using the word.