17. officer, official

officer部隊的軍官;
official政府官員 eg:an army officer

18. work, job

二者均指工作。work不可數，job可數 eg:a good job

19. couple, pair

couple主要指人或動物;
pair多指由兩部分組成的東西 eg:a pair of trousers

20. country, nation, state, land

country側重指版圖，疆域;
nation指人民，國民，民族;
state側重指政府，政體;
land國土，國家 eg:The whole nation was sad at the news.