REVIEW OF VERB TENSES

PRESENT PERFECT
When we talk about something that happened in the past, but we don´t specify precisely when it happened( perhaps we don´t know, or it is not important to say when it happened), we use the present perfect:
I have already traveled to Japan.
Mom has already done three surgeries.
My boyfriend has already had three car accidents.

PAST SIMPLE
When we want to indicate that something happened at a specific time in the past, we use the past simple. We can either say when it happened, using a time adverb, or assume that the listener knows when it happened or can understand this from the context:
I arrived late for the meeting yesterday.
I went to the movies alone last weekend.
Mom gave me two gifts on my last birthday.

We use the present perfect when we talk about something that happened in a period of time up to the present. We use the past simple to talk about something that happened at a particular, finished time in the past.
Science has made many advances this century.
Scientists made some fundamental discoveries in the 18th century.

PRESENT PERFECT CONTINUOUS
We use the present perfect continuous to talk about a situation or activity that started in the past and has been in progress for a period until now. Sometimes we use the present perfect continuous with expresssions that indicate the time period (with since and for):
Jane has been living in New York for over a year now.
I have been working as a teacher since last year.
John has been exercising a lot because he needs to loose weight.

PAST PERFECT
We use the past perfect to talk about a situation or activity that happened before another past situation or activity, or before a particular time in the past:
Jo discovered that Leslie had lied to her.
________________________________
Leslie lied Jo discovered }past now

We use the past simple rather than the past perfect when we simply talk about a single activity or event in the past:
I handed the letter to him.
I´m late because I took the wrong bus.

Notice the difference in meaning of these sentences with the past perfect and past simple:
When he stopped laughing, everyone left. (they left after he stopped laughing)
When he stopped laughing, everyone had left. (they left before he stopped laughing)
I got up when the phone rang. (the phone rang and then I got up)
I had gone to bed when the phone rang. (I went to bed and then the phone rang)