这四个说法都是往回看，指的是已经过去的时间。Yesterday 是今天的前一天。Last night 是今天早上之前的那个晚上。Last week 是这个星期的前一个星期。Ago 的用法不一样：它放在一段时间的后面，从现在往回数：an hour ago、three days ago。
过去的时间：yesterday, last night, last week, ago
yesterday /ˈjestərdeɪ/ — The park was very busy yesterday.
last night /ˌlæst ˈnaɪt/ — We were at home last night.
last week /ˌlæst ˈwiːk/ — Camila was in Ghafwick last week.
ago /əˈɡoʊ/ — The film was two hours ago.
试一试: Mei: Is the English test today? Lars: No, you have missed it by one day. It was ___. 补全 Lars 说的话。 (yesterday / last week / last month) 答案: yesterday — Lars 说 Mei 晚了一天，所以考试是在今天的前一天。另外两个答案都比一天早得多。
试一试: Ira: You look very tired this morning. Diego: I know. I was still awake at two o'clock ___. 补全 Diego 说的话。 (last night / last month / this week) 答案: last night — Diego 两点的时候还醒着，而 Ira 是在今天早上跟他说话，所以他说的两点就是刚过去的那个夜里。另外两个答案说的根本不是某个晚上。
试一试: Amina: Today is Tuesday. Is the book club this week? Sofia: No, it was on Friday, ___. 补全 Sofia 说的话。 (last week / this week / last night) 答案: last week — Amina 说今天是星期二，所以这周的星期五还没到。Sofia 说的星期五，是这周开始之前的那一个。而刚过去的那个晚上是星期一晚上，不是星期五。
试一试: Lars: Is Ethan still at the bank? I need him now. Mei: He was there ___, so he is very near. 补全 Mei 说的话。 (ten minutes ago / last night / last month) 答案: ten minutes ago — Mei 知道 Ethan 就在附近，所以他几分钟前还在银行。另外两个答案都比这早得多，也没法告诉 Lars Ethan 现在在哪儿。