Use the PRESENT SIMPLE to describe:

• A regular or habitual action.
  She usually takes the train to work.
  We often use adverbs or time expressions of frequency with this use of present simple: often, usually, sometimes, once a week, twice a month.

• A factor general truth.
  Many people don’t believe in horoscopes
  Bill Clinton spends most of his time in the USA.

Use the PRESENT CONTINUOUS to describe:

• An action happening around now (often temporary).
  Sorry, she’s speaking to someone at the moment.
  We’re currently looking for new people.

• A trend or changing situation.
  Prices are rising steadily at present.
  More and more people are learning English.

• A photograph or a scene.
  In the photograph, the two people are talking.

Use the PAST SIMPLE to refer to finished actions that are in the past.
I began school in 1989.
He set out across Europe in 1271.
We often use the past simple when we know or say the exact time of the action and with time expressions such as yesterday, last week, in 1271.

Use the PRESENT PERFECT to talk about:

• Finished actions in a time period that continues up to now. We often use time expressions such as today, this week, this year.
  We’ve been really busy all week.
  You’ve worked hard all year.

• Experiences in our lives when we don’t say when they happened. We often use adverbs such as never, ever, already, yet, just.
  Have you ever visited their house?
  They’ve never climbed Mont Blanc.

ALREADY, YET and EVER

These tree adverbs are often used in conjunction with the present perfect:

• Ever is used in questions to refer to any time up to the present.
  Have you ever been Germany?

• Already is used in affirmative sentences and questions to refer to an event close to the present that has happened.
  We’re already done that. What’s next?

• Yet is used in question and negative sentences to refer to an event that is planned or expected, but not completed.
  I haven’t had time to do it yet.

We use already and ever before the main verb, but yet comes at the end of the sentence.
I have already finished it.
Sorry, I haven’t finished it yet.

PRESENT PERFECT and PAST SIMPLE

Present perfect and past simple both refer to the past, but the present perfect doesn’t necessarily say or imply when something happened.
We’ve already found someone suitable.

The present perfect and past simple can both refer to finished actions, but the past simple refers to a specific past time and the present perfect to a time continuing up to now.
We looked for someone suitable last year.
We’ve looked for someone suitable this year.

We rarely use adverbs such as ever, already, yet with the past simple.

PRESENT PERFECT CONTINUOUS

We use the present perfect continuous to talk about actions and situations that continue into the present.
I’ve been studying at Oxford University for two years.
(and I am still studying there)
They’ve been talking on the phone for two hours.
(and they are still talking)

We often use the present perfect continuous with the time expressions since and for. Use since to refer to a point in time and for to indicate a period of time.
I’ve been studying at Oxford University since 2006.
I’ve been studying at Oxford University for two years.

We don’t usually use state verbs with the continuous form.
I’ve been knowning John for years.
I’ve known John for years.

We use the PRESENT PERFECT SIMPLE to emphasise:

• That an action started in the past and is completed.
  I’ve prepared the leaflets you asked for.
  They’ve interviewed everyone.

• The result of the activity (for example, by telling us how much or how many).
  We’ve asked over five hundred people.
  He’s done all his homework.
  How many people have you interviewed?
  How much of your work have you done?

We use PRESENT PERFECT CONTINUOUS to emphasise:

• That an action started in the past and is still continuing.
  She’s been working here for over three years.
  We’ve been writing letters to all the candidates.

• The activity and its duration (for example, by telling us how long).
  Roger has been playing tennis for years.
  Roger has been playing tennis since he was a boy.
  How long have you been working here?

Use the PAST CONTINUOUS

• To talk about background actions.
  It was a beautiful day — the sun was shining and the birds were singing.

• To talk about a longer background action in the past when a shorter action interrupts it or happens during it.
  We were talking about you when you rang.
  (the background action is interrupted and stops).
  
  
  
  
  
  
  
  
  I was having a staf meeting when Julia arrived five minutes late.
  (the action happens during the background action)

• To talk about repeated actions in the past that take place over a temporary period of time.
  People were coming in all day to buy this item.

• To ephasise the uration or continuity of a past action.
  For the whole of last month we were trying to solve this same problem.

We rarely use state verbs such as be, like, know, believe, understand in the continuous form.
I was liking the fish soup.
I liked the fish soup.

PAST PERFECT SIMPLE

Use the Past perfect simple to emphasise that one action happened before another in the past.
The film had already started when we arrived at the cinema.

The past perfect simple is often used with the conjunctions: when, before, after and by.
I had left when he called.
She had been to Australia twice before we went together.
We went out after we had eaten breakfast.
By the age of 80, he had written over a hundred books.

If the sequence of events is clear, we can also use the past simple.
I called my friend after I finished work.