To Sink
To Go Down, To Become Depressed
intransitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means sink so the verb vocab version is to sink.
On a literal level, 沈む refers to physical sinking — a boat sinking into the water or someone's foot sinking into the mud, for example. However, it can also be used metaphorically to talk about things like the sun setting (太陽が沈む), a nation falling into decline (国が沈む), or even someone "sinking" into a depressed mood (気分が沈む).
Your boat is about to sink, so you run around yelling, "She's (しず) sinking! She's… she's sinking! Abandon ship!" Boats are always female, so be careful not to say "it." If you don't say "she's" sinking, your crew might not get the message.
船はゆっくりと海に沈んでいった。
The ship slowly sank into the sea.
夕日が沈むと、街に明かりがともった。
When the sun set, lights came on in the city.
なんだか沈んだ顔をしているけど、何を思い詰めてるの？
You look a bit depressed. What's on your mind?
カフェラテの泡が沈んでいくのを見て、人生みたいだなと思った。
Watching the foam sink in my latte, I thought, "That's kinda how life is."