which one of following sentences is better? You shouldn't rinse/rinse out your mouth after using a mouthwash. I am very likely to say "rinse", but since "rinse out" is good too, I am a bit confused.
"Rinse" is fine by itself, Gandalf. Native English-speakers can turn just about any verb into some sort of phrasal verb, which is probably why "rinse out" exists even though it doesn't add anything to the meaning of "rinse" in this context. You'll find many similar cases in English. When you run across variants that mean the same thing, feel free to choose whichever one you prefer.