tense (時態) 的觀念
1. 過去式 (past tense): 如果句子裡有很明顯表示過去特定時間的副詞片語(e.g. yesterday, last night, two days ago, in 2005)，幾乎可以確定要用過去式。
錯誤 Ex. Mary
walks downtown yesterday.
正確 Ex. Mary walked downtown yesterday.
2. 表示過去的習慣動作時用過去式(past tense)，經常與時間副詞如: every day, seldom, from time to time, usually 等連用。
Ex. Last week, I worked until midnight every night.
Ex. He seldom felt lonesome while he was traveling.
Ex. We saw him from time to time.
3. 現在完成式 (present perfect tense)的公式: have/has +過去分詞 (如果不確定過去分詞的拼法，請查字典)，表示現在剛剛完成之動作，常有下列副詞:
副詞: already, yet, just, recently, lately, today, this week, this morning, this afternoon 等
Ex. I have seen him this morning.
Ex. We have been busy this afternoon.
Ex. I have just finished my homework.
Ex. He has bought a new car recently.
4. 現在完成式 (present perfect tense) 表示過去繼續到現在的動作或狀態，常用for, since所引導的副詞表示持續之期間，也常與How long連用。
since she was nine years old; since he
graduated from high school; since last month;
for + 一段時間 (e.g. for three years; for five months; for hours等)
Ex. Margaret has studied ballet since she was a child.
Ex. I have collected coins for five years.
Ex. Betty has taught
for ten years. (表示Betty還在教書)
Ex. Betty taught for ten years. (表示Betty已經退休沒有教書了)
5. 現在完成式 (present perfect tense) 表示過去某時發生的動作，其結果影響到現在，或其狀態持續到現在。(Present Perfect refers to completed actions which endure to the present or whose effects are still relevant. )
Ex. He has eaten nothing today. (他今天什麼都沒吃 -- 他現在一定很餓。)
Ex. I have learned the lesson by heart. (我已經熟記下這一課 -- 我現在可以背誦了。)
Ex. I have lost my watch. (我手錶丟了 -- 我到現在還沒有找到)
6. 過去完成式 (past perfect tense)的公式: had +過去分詞 (如果不確定過去分詞的拼法，請查字典)，經常與過去簡單式或表示過去的時間副詞一起用，來表示其發生的時間比過去簡單式早。
Ex. I didn’t go to the film with Mary because I had already seen it. (我當時沒有和Mary去看電影，因為我已經看過了)
Ex. He had been ill for a week when he was sent to the hospital. (當他被送進醫院就醫的時候，已經病了一個星期了)
Ex. Renee had washed
the car when George arrived. =
She had already finished washing the car by the
time he arrived.
Ex. Renee washed the car when George arrived. = She waited until George arrived and then washed the car.
7. 過去進行式 (past progressive tense)的公式: was/were + V-ing，表示過去某一時候正在進行的動作。通常過去進行式就像是發生在過去的背景活動，這背景活動發生期間，另外一件事件發生用過去簡單式。
Ex. She was studying when the lights went out.
Ex. While I was studying, I fell asleep.
when = at that time; while
= during that time
Ex. When the telephone rang, I was taking a shower.
Ex. While I was taking a shower, the telephone rang.
Ex. I sat down at the dinner table at 6:00 p.m. yesterday. Tom came to my house at 6:10 p.m. I was eating dinner when Tom came.
8. 時態順序的一致 (sequence of tense)也是台灣學生常犯的錯誤，許多初學者在時態使用常常主要子句用過去式，後面的附屬子句突然就變成現在式。或者，明明是在描述過去發生的事情，原本是用過去式，突然下一句就變成現在式，通常這樣的句子是錯誤的。一般的原則是: 主要子句用過去式，後面附屬子句的動詞也要用過去式。如果是描述過去發生的事情，大部份情況都是用過去式。
Ex. I knew that he had a fever.
Ex. I could see that he was sick.
Ex. He told me that he owned property in Mexico, but that was a lie.
Ex. I know that he works
I know that he is working hard.
I know that he has worked hard.
I know that he will work hard.
10. 主要子句是過去式，如果附屬子句說的是「真理或不變的事實」(eternal truth)，則可以用現在式。
Ex. We were taught at school that water consists of hydrogen and oxygen.
Ex. Our teacher told us that honesty is the best policy.
Note: 主要子句是過去式，如果附屬子句說的是「歷史的事實」(historical truth)，則用過去式。
Ex. We learned that America was discovered by Columbus in 1492.
11. 唉!通常有規則或原則就是會有例外。主要子句是過去式，附屬子句如果說的是「至今仍未改變的習慣」，附屬子句的訊息「現在依然是真實的」(current validity)，而且「現在依然很重要」(is also of importance at the present moment)則可用現在式或現在進行式。
Ex. He said he brushes his teeth after every meal. (他說他每餐飯後都刷牙)
Ex. He said that he usually reads the newspaper while he is waiting for the bus. (他說他等公車時經常看報紙)
Ex. He told his brother that I am always finding fault with him. (他告訴他哥哥說我總是挑他的毛病)
found out that he’s now a doctor and that
he’s practicing in
Ex. He said he
gets up at six o'clock every morning.
Ex. He said he got up at six o'clock every morning. (他過去都是早上六點起床，但此習慣已經改變)
Ex. He told me just now that he is going to England next month. (他剛剛告訴我，他下個月要去英國) [尚未出發]
Ex. He said just a moment ago that he will (不是would) come to my house tomorrow. (他剛剛告訴我，明天要來我家)