Now that you're studying English at a higher level, you need to be able to talk about the language itself.
One of the biggest areas of confusion for students is the difference between the past tense时态 and the present perfect完成时.
The past tense is indicated by use of the past tense marker, represented here by V sub d.
Here are some examples of regular verbs and their past tense forms.
V V（d）
walk walked
wait waited
arrive arrived
open opened
close closed
So, as you can see, for regular verbs the past tense form is indicated by the addition of an -ed at the end of the verb.
However, some of the most common verbs in English are irregular, so it is important to learn them.
Here are some examples.
V V（d）
eat ate
drink drank
get got
take took
come came
be was/were
Altogether there are over 200 irregular verbs in normal use.
Many people think that the past tense form only indicates past time.
However, that is only one of the possible meanings.
The past tense form is also used to express something that is unlikely or imaginary, from the speaker's point of view.
Here is an example: "He wishes he had a car."
The fact is that he doesn't have a car, so the use of "had" indicates that it isn't real: it's imaginary.
“If he had a car, he could drive us to school.”
Again, "he doesn't have a car, so he can't drive us to school."
This condition expresses something that isn't a fact.
It's a counter-factual condition, which means it goes against what we know or believe.
Look at these two sentences.
If she waits another week, she can save money…
If she waited another week, she could save money because there’s going to be a sale.
Both sentences are correct, but there is a difference in meaning.
In the top sentence, the condition that she waits is more likely from the speaker's point of view.
In the bottom sentence, the speaker thinks it's less likely that she's going to wait.
(He wishes he had a car,but he doesn't have enough money to buy one.)
(There was a big sale last week, so we all went shopping.)
(There is going to be a sale next week, so I hope she doesn’t go shopping tomorrow.)
(If that flight leaves on time, we’re going to miss it, so let’s run.)
(If that flight left a few minutes later, we could take the next shuttle bus.)
(I wish we didn’t have to run, but if we don’t hurry, we’ll miss the next bus.)
(If that flight leaves on time, the next shuttle bus may not get us to the airport on time.)
(If that flight left an hour later, we wouldn’t have to get to the airport so early tomorrow morning.)
When we talk about experience or results, we use the present perfect form.
This form uses "have" followed by the participle.
For regular verbs, this participle分词 is identical to the past tense form.
V V（d） V(n)
walk walked walked
wait waited waited
arrive arrived arrived
open opened opened
close closed closed
For irregular verbs, this participle is often different.
V V（d） V(n)
eat ate eaten
drink drank drunk
get got got
take took taken
come came come
be was/were been
When we use the perfect form, the primary focus is on the subject of the sentences, not the verb.
It emphasizes强调 condition, experience or results rather than actions or events.
In these two examples, we see the contrast.
She has already left the hotel.
She left the hotel a few minutes ago.
The top sentence gives the condition or state of the subject and the bottom sentence expresses an event.
The top sentence describes the condition of the subject of the sentence, she, rather than an event.
The fact that she has left the hotel is now a part of her experience, or state of being.
The bottom sentence expresses the action, or event: "she left," which is something she did.
In general, the past tense form expresses of events or acts, and the perfect form expresses the experience or conditions.
When we use the perfect form of the verb we focus on the state of the subject and not an action or event.
(In high school, John wasn’t a very good student.)
(In fact, he was one of the worst students.)
(When it came time to go to university, nobody expected him to pass the entrance examination.)
(However, he proved them wrong and did well enough to get into a top university.)
(He enjoyed his studies and graduated a few months ago.)
(Now that he has graduated, he’s ready to get a job.)
(He has practiced hard…so he should do well in the competition.)
(He may have been injured in the explosion.)
(The game has started already.)
(He may have taken his computer with him, but I’m not sure if he did or not.)
(He may take his computer with him, but he still hasn’t decided.)
(His computer may have been taken by somebody.)
(One of the biggest areas of confusion for students is the difference between the past tense and the present perfect.)
Note that the past form expresses events at specific points or periods of time, such as last year or the past five years.
He worked at several companies(in the past)
He finished the project last week.(in the past, last week)
These events are distanced from the speaker's point of view.
The perfect form expresses the state of something from the speaker's point of view, not distanced in time.
He has worked at several companies.(his experience now)
He has finished the project.(his experience now)
These two sentences are incorrect in confusing, because they give two different points of view.
He has worked at that company until last week.(incorrect)
He has finished the project last week.(incorrect and confusing)
Here are some sentences that use the perfect form.
He has finished practicing.
He has been practicing.
She may have eaten.
Her computer may have been stolen.
Some are simple, such as the first one, and some are complex, like the last one.
The last one is complex because it combines many verb forms,including the passive.
He took his computer with him because he wants to work at home.
He may have been traveling when he got sick.
He must have taken her computer with him.
He may take his computer with him, but he still hasn't decused.
It might have been broken by accident.
(He may have been arrested by the police.)
(His computer was taken in for repair last week,so it should be ready soon.)
(The fire might have been started by someone.)
(It has been getting warmer lately.)