how long 后面怎么加 for two days 和 since two days ago 到底怎么加 什么时候加

how long 后面怎么加 for two days 和 since two days ago 到底怎么加 什么时候加 another two days 后面加is还是are since two day ago和two day ago还是for two day相同?怎么区分 the days 什么时候后面加 that 什么时候加 when? It started (about two days ago ).对加括号的部分提问.（ ） （ ）it（ Mr Green left Shanghai two days ago （用since two days ago改写 two meters long中的meter为什么直接加s 而two days' holiday 中的day为什么变所有 two days 和 for two days 有什么区别 He has left Beijing since two days ago改错 Tom __________(be) away since two days ago. She has fallen ill since two days ago.(改错) He has left Beijing since two days ago.改错