1. The simple past is used to talk about completed actions in the past

2. The simple past is often used with expressions that refer to points of time in the past
 
 

at	4 o'clock/2.12/the end of year/Christmas
on	Tuesday/19th March/the 21st/New Year's Day
in	January/1999/the 1990s/summer
no prepositions	yesterday/yesterday morning/last Monday/next April/a few days ago/ the day before yesterday/when I was young

 Present perfect simple

3. Present perfect is used to talk about a present situation which is a result of something that happened at an unspecified time in the past.
Therefore we do not use specific time expression such as yesterday, last week, etc.

I have given your article about networking to professor.
(I gave him your article and he has it now.)

You cannot meet Mrs Jones. She has just left.

I am afraid Mrs. Jones as not here at the moment. She has gone to the meeting in Brussels.
(She is still at the meeting.)
Anita has been to the travel agent. She has her tickets for USA.
(She went to the travel agent and has returned.)

Have you ever worked abroad.
(i.e., In all your life up to now?)
I have never been to China.
(i.e., Not in all your life up to now.)

Have you ever been in Malaysia?
Yes, I have. I was in Kuala Lumpur at INET'97 when I worked in KPI.

Already is used in positive sentences. It often indicates that something has taken place slightly earlier that expected.

She has already printed this page.
(Note. NOT: She has  printed already...)

Have you talked to Peter yet?
(Note. NOT: Have you talked yet to Peter?)
I have not talked to him yet.
(Note. NOT: I have not talked yet to him.)

This week we have received a lot of enquiries about our new web site.
(The week has not finished yet, and there may be more enquiries.)

Have you seen John this morning?
(It is now 10.30 in the morning; and the morning has not finished.)
Did you see John this morning.
(It is now 3.00 in the afternoon; the morning has finished.)

The present perfect simple is often used with for and since and stative verbs to talk about things that began in the past and have continued up to now.

I have known about the plans to spin off this service from the company. (And I know now.)

I have been with my department for three years.
I have been in Internet technologies since 1992.

To ask questions about periods of time, we can use How long ...? + the present perfect

How long have you been in Amsterdam? I have been here since September / for six months.

WRONG: I am here since January.
RIGHT: I have been here since January.

We can use the present perfect negative to talk about the amount of time that has passed between now and last time something happened.

We haven't received any messages from him for several months / since the last working group meeting.

If we talk about a completed action (particularly if we give details about how much, how many, etc.), we can use the present perfect and since (but not for). We can also use other phrases of duration such as to date, recently, over the past two years, etc. The action itself is finished, but the period of time extends up to the present.

The Commission has launched three new programs since December.

13. Unfinished activities

Present perfect continuous is used with for, since, and How long ...? and other expressions of duration (e.g., all month), to talk about activities that started happening in the past and are still happening now. The activity may have been going on continuously or repeated several times.

They have been coordinating network development for 5 years.
(They started coordinating 5 years ago . They are still coordinating network developement.)

I have lived in Kiev all my life (NOT: I have been living ...)

We use the present perfect simple if we are talking about a completed action, particularly if we give details of how much or how many. we use the present perfect continuous when something is still going on.

I've written a report for Peter. (It is finished.)
I've been writing a report about international characters usage. (I am still writing it.)

In the negative, the focus on the present perfect simple is on the amount of time that has passed since something happened. The focus of the present perfect continuous is on the verb itself.

I haven't met him for six months. (The last time was six months ago.)
I haven't been feeling well recently. (This has been continuing for days.)

We use present perfect continuous to talk about an activity that was in progress, but has just finished. Normally there is some evidence.

The ground is very wet. It has been raining.

Summary of Usage:
Simple Past vs Present Perfect vs Present Perfect Continuous
 

The Simple Past	We normally use the simple past to talk about actions that took place at a time that is separated from the present. It is used with expressions like yesterday, on Monday, last week, in 1998, etc.	Last month Vodafon launched a takeover bid for Airtouch. He did his PhD at Delft Technical University.
	We can use the simple past and for to talk about something that happened during a period that has now finished.	He lived in Amsterdam for five years; then he came back to England.
The Present Perfect Simple	The present perfect is used to talk about the present result of past actions and recent events, and often used with words like ever, never, just, already, yet, and phrases of unfinished time such as so far.	A2000 has cut installation price for Internet over TV cable network by 30%. Have you ever tried Swiss wine.
The Present Perfect Simple + for and since	The present perfect can be used with for and since and stative verbs, or to refer to actions that are seen as long term or permanent.  We use for to talk about the duration of the period of time and since to talk about the starting point of an action or state.	I have been with my department for three years. I have been in Internet technologies since 1992.
	It is also used in the negatives with for and since to talk about the last time something took place	I haven't met him for six month. I haven't been feeling well recently.
	It is used with since to talk about completed action.	The Commission has launched three new programs since December.
The Present Perfect Continuous	The present perfect continuous can be used with for and since to talk about activities that have gone on repeatedly or continuously for a period of time, and are still going on.	The Commission has been coordinating network development for 5 years.