The present perfect tense identifies an action or a state that took place at an unspecified time in the past. The present perfect progressive expresses an ongoing activity that started in the past and continues into the present.

Examples

I have been to that city. (present perfect)

It has been drifting through my mind lately. (present perfect progressive)

Source: Have Been vs. Has Been vs. Had Been: How to Use Each One Correctly – The Blue Book of Grammar and Punctuation

The simple past is used for a single event (or sequence of such events) in the past, and also for past habitual action:

He took the money and ran.
I visited them every day for a year.

It can also refer to a past state:

I knew how to fight even as a child.

For action that was ongoing at the time referred to, the past progressive is generally used instead (e.g. I was cooking). The same can apply to states, if temporary (e.g. the ball was lying on the sidewalk), but some stative verbs do not generally use the progressive aspect at all – see Uses of English verb forms § Progressive – and in these cases the simple past is used even for a temporary state:

The dog was in its kennel.
I felt cold.

However, with verbs of sensing, it is common in such circumstances to use could see in place of saw, could hear in place of heard, etc. For more on this, see can see.

If one action interrupts another, then it is usual for the interrupted (ongoing) action to be expressed with the past progressive, and the action that interrupted it to be in the simple past:

Your mother called while you were cooking.

The simple past is often close in meaning to the present perfect. The simple past is used when the event happened at a particular time in the past, or during a period which ended in the past (i.e. a period that does not last up until the present time). This time frame may be explicitly stated, or implicit in the context (for example the past tense is often used when describing a sequence of past events).

I was born in 1980.
We turned the oven off two minutes ago.
I came home at 6 o’clock.
When did they get married?
We wrote two letters this morning.
She placed the letter on the table, sighed, and left the house.

These examples can be contrasted with those given at Uses of English verb forms § Present perfect. Also, for past actions that occurred before the relevant past time frame, the past perfect is used.

Various compound constructions exist for denoting past habitual action. The sentence When I was young, I played football every Saturday might alternatively be phrased using used to (… I used to play …) or using would (… I would play…).

The simple past form also has some uses in which it does not refer to a past time. These are generally in condition clauses and some other dependent clauses referring to hypothetical circumstances, as well as certain expressions of wish:

If he walked faster, he would get home earlier.
I wish I knew what his name was.
I would rather she wore a longer dress.

For more details see the sections on conditionals, dependent clauses and expressions of wish in the article on uses of English verb forms.

For use of the simple past (and other past tense forms) in indirect speech, see Uses of English verb forms § Indirect speech. An example:

He said he wanted to go on the slide.
Source: Simple past – Wikipedia

VERBS

Source: Verbs | English Grammar | EF

This analysis of conditional verb forms was written by Rob De Decker, who teaches English at a Flemish grammar school (equivalent to an American high school) in Schellebelle, Belgium. It is used here with his permission.

Conditional Clause and Main Clause

If I have enough money, conditional clause	I will go to Japan.     main clause
I will go to Japan, main clause	if I have enough money     conditional clause

First, Second, and Third Conditional

1. First conditional:	If I have enough money, I will go to Japan.
2. Second conditional:	If I had enough money, I would go to Japan.
3. Third conditional:	If I had had enough money, I would have gone to Japan.

 

Conditional clause	Main clause
1. If + Present Tense	will + inf / present tense / imperative
If you help me with the dishes (if + pres),  I will help you with your homework. (will + inf) If the sum of the digits of a number is divisible by three,  the number is divisible by three (Pres. tense) If you see Mr Fox tonight, tell him I am ill. (imperative).
2. If + Past Tense	would + inf
3. If + Past Perfect Tense	would have + past participle
We do not normally use will or would in the conditional clause,  only in the main clause.

Uses of the Conditional

1. First conditional
   1. Nature: Open condition, what is said in the condition is possible.
   2. Time: This condition refers either to present or to future time.
      e.g. If he is late, we will have to go without him.
      If my mother knows about this, we are in serious trouble.

2. Second conditional
   1. Nature: unreal (impossible) or improbable situations.
   2. Time: present; the TENSE is past, but we are talking about the present, now.
      e.g. If I knew her name, I would tell you.
      If I were you, I would tell my father.
      Compare: If I become president, I will change the social security system. (Said by a presidential candidate)
      If I became president, I would change the social security system. (Said by a schoolboy: improbable)
      If we win this match, we are qualified for the semifinals.
      If I won a million pounds, I would stop teaching. (improbable)

3. Third conditional
   1. Nature: unreal
   2. Time: Past (so we are talking about a situation that was not so in the past.)
      e.g. If you had warned me, I would not have told your father about that party.(But you didn’t, and I have).

Remember!

1. The conditional construction does not normally use will or would in if-clauses. EXCEPTION: If will or would express willingness, as in requests, they can be used in if-clauses.

e.g. If you will come this way, the manager will see you now.
I would be grateful if you would give me a little help.
(= ± please, come this way; please, give me…)

2. For the second conditional, were replaces was:

If I were a rich man…

3. After if, we can either use “some(-one, -where…)” or “any(-one, -where…).

If I have some spare time next weekend….or : 
If I have any spare time…

4. Instead of if not, we can use unless.

e.g. I’ll be back tomorrow unless there is a plane strike.
He’ll accept the job unless the salary is too low.

5.There is a “mixed type” as well, for the present results of an unreal condition in the past:

If + Past Perfect – would + inf.
If you had warned me [then], I would not be in prison [now].

 

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Guide to Grammar and Writing

Principles of Composition

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Source: First, Second, and Third Conditional