Verb tenses can indicate the past, present, and the future, as well as a more complicated mixture of time. Although there are some who would argue that there can be as many as thirty different verb tenses, these are for the most part auxiliaries. There are only six basic tenses that allow you to re-create the reality of time in your writing.
Use the simple present to express the idea that an action is repeated or usual. It can also indicate the speaker believes that a fact was true before, is true now, and will be true in the future, or to talk about scheduled events in the near future. Sometimes it is used to make generalizations about people or things.
He plays golf. I do not play golf. Does she play golf?
Cats like milk. Birds do not like milk. Do pigs like milk?
The bus leaves every morning at eight. The bus does not leave at nine. When does the bus usually leave?
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Present perfect is used to say that an action happened at an unspecified time before now. The exact time is not important. You do not use the present perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in France, at that moment, that day, one day, etc. You can, however use the present perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc.
"Last year" and "in the last year" have very different meanings. "Last year" means the year before now, and it is considered a specific time which requires simple past. "In the last year" means from 365 days ago until now. It is not considered a specific time, so it requires present perfect.
Did you go to Mexico last year? (simple past)
Have you been to Mexico in the last year? (present perfect)
You can use this tense to describe an experience or to say that you have never had a certain experience, but not to describe a specific event. The present perfect is often used to talk about change that has happened over a period of time or to list accomplishments.
I have been to Mexico. I have never been to Mexico.
Judy has grown over the last year.
Doctors have cured many deadly diseases.
Present perfect can be used to suggest we are waiting for an action to happen or that the process is not complete and more actions are possible.
The package has still not arrived.
I have had many problems while working on this novel.
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We use the simple past to express the idea that an action started and finished at a specific time in the past. It can be a single action, or a series of actions.
I saw my mother yesterday.
I came home from work, took my shoes off, had a drink, and made supper.
This tense is also used to describe a habit which stopped in the past, or something which starts and stops in the past.
I smoked when I was younger. (similar to - I used to smoke.)
I smoked for five years.
Simple past is also used to describe past facts or generalizations which are no longer true. As in the first of above examples, this use of the simple past is similar to the expression "used to."
He was skinny as a boy, but now he has filled out. (He used to be skinny)
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The past perfect expresses the idea that something occurred before another action in the past. It can also show that something happened before a specific time in the past.
I could not pay my way because I lost my wallet.
I had never been to a concert before last week.
Past perfect is used to show that something started in the past and continued up until another action in the past.
I had that lap top for five years before it got a virus.
With past perfect it is possible to use specific time words or phrases although it’s not usually necessary.
She had visited her Maritime relatives once in 1999 before she moved in with them in 2001.
If the action did occur at a specific time, the simple past can be used instead of the past perfect when "before" or "after" is used in the sentence. For this reason, both examples below are correct:
She had visited her Maritime relatives once in 1999 before she moved in with them in 2001. (past perfect)
She visited her Maritime relatives once in 1999 before she moved in with them in 2001. (simple past)
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Simple future has two different forms: "will" and "be going to." Although the two forms can sometimes be used interchangeably, they often express two very different meanings.
"Will" suggests that a speaker will do something voluntarily. Often, we use "will" to respond to someone else's complaint or request for help. We also use "will" when we want someone to help us or volunteer to do something for us. We use "will not" or "won't" when we refuse to voluntarily do something.
I will forward the letter when I get it.
I will help clean up, so Mom doesn’t find out.
I will not do the dishes.
"Will" is usually used in promises. "Going to" expresses that something is a plan, that a person intends to do something in the future. It does not matter whether the plan is realistic or not.
I will do what I can.
He is going to spend all is money on an iPad.
Both "will" and "be going to" can express the idea of a general prediction about the future. Predictions are guesses about what might happen. In these types of sentences, the subject usually has little control over the future.
This will be a very interesting book.
This is going to be a very interesting book.
Simple future cannot be used in clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc. Instead of simple future, simple present is used.
When you will arrive tonight, we will go out for dinner. (incorrect)
When you arrive tonight, we will go out for dinner. (correct)
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Future perfect has two different forms: "will have" and "be going to." Future perfect forms are usually interchangeable, unlike simple future forms.
You will have perfected your technique by the time you finish your novel.
You are going to have perfected your technique by the time you finish your novel.
Are you going to have perfected your technique by the time you finish your novel?
The future perfect expresses the idea that something will occur before another action in the future. It can also show that something will happen before a specific time in the future.
By the time the guests arrive, I am going to have cleaned the entire house.
By next week, I will have finished my novel.
We use the future perfect to show that something will continue up until another action in the future.
I will have been in the library for six hours by the time I leave.
By Monday, Susan is going to have had my book for a week.
Like all future forms, the future perfect cannot be used in clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc. Instead of future perfect, present perfect is used.
I am going to buy a book when I will have saved enough money. (incorrect)
I am going to buy a book when I have saved enough money. (correct)