To Flush Something, To Pour Something, To Set Something Afloat
transitive verb, godan verb
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means stream. If you're streaming water, it might be to flush something. Alternatively, to pour something makes a stream of liquid, and to set something afloat is to make something flow in the stream.
ながす
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: What do you do with someone who is a nagger (one who nags)? You have to flush them down the toilet. Imagine someone nagging you to do your chores so you flush them down the toilet, somehow.
トイレは使ったら水を流しましょう。
Flush the toilet after using it.
そうめんを竹に流して食べるのが「流しそうめん」です。
"Nagashi somen" is where you flow somen noodles through a bamboo tube and eat it.
毎日汗を流して働く労働者たちに感謝の気持ちもないのか？
Are you not even grateful to the workers who sweat and work every day?