How do you say “How is the weather in Seoul?” in Korean?

= 서울 날씨 어때요? [seo-ul nal-ssi eo-ttae-yo?]

How do you say "What would you recommend on the menu?" or "What are the popular dishes at your restaurant?" in Korean?

= 뭐가 제일 인기 많아요? / 뭐가 제일 잘 나가요?

[mwo-ga je-il in-kki ma-na-yo?] / [mwo-ga je-il jal na-ga-yo?]

How do you say "Is this sentence grammatically correct?" in Korean?

= 이 문장(이) 문법에/문법적으로 맞아요? [i mun-jang-(i) mun-bbeo-be/mun-bbeop-jeo-geu-ro ma-ja-yo?]

How do you say “bathroom” in Korean?

= 화장실 [hwa-jang-sil]

How do you say “Stop laughing. It's not funny.” in Korean?

= 그만 웃어. / 웃지 마. [geu-man u-seo.] / [ut-ji ma.]

재미 없어. / 안 웃겨. [jae-mi eop-sseo.] / [an ut-gyeo.]