“습니다/ㅂ니다” is use after a verb/adjective, to end the sentence with a much formal expression. Likewise, to end a question formally, “습니까?/ㅂ니까?” is used.
To choose “습니다/습니까” or “ㅂ니다/ㅂ니까” will depends on whether the last word has a final consonant or not.
“웃다 ” has a final consonant, so we’ll use “습니다/습니까”.
웃다 + 습니다/습니까 = 웃습니다 / 웃습니까?
“자다” does not have final consonant, hence, “ㅂ니다/ㅂ니까” is used.
자다 + ㅂ니다/ㅂ니까 = 잡니다 / 집니까?
아이가 웃습니다 / 아이가 웃습니까? The kid laughed / Did the kid laugh?
동생이 잡니다 / 동생이 집니까? Brother has fell asleep / Has brother fell asleep?
손이 큽니다 / 손이 큽니까? The hand is big / Is the hand big?
When this consonant “ㄹ” meets “-습니다/ㅂ니다/습니까?/ㅂ니까?”, our little friend “ㄹ” will disappear.
살다 >> 삽니다
알다 >> 압니다
팔다 >> 팝니다
멀다 >> 멉니다
길다 >> 깁니다
Above is from a chinese source (http://kr.hujiang.com/new/p171155/) and I tried my best to translate into English in order to share with our non-Chinese friends. Hope it helps. ^^
– 크리스티 ✿