The use of conditional patterns in English is rather complicated.  What follows by no means exhausts the subject, but is intended only to explain and give examples of some of the more common patterns.

Certain words or phrases, like if, provided that, so long as, unless *, etc. introduce subordinate clauses that express a condition.  The main clause then tells us what will or would happen, is or would be happening, has or would have happened, as a result of the fulfillment or non fulfillment of the condition.
That is, conditions may be expressed concerning future, present or past time.

*Unless 

Example:

• Unless you study, you won’t learn.
• If you don’t study, you won’t learn.
• If you study, you will pass the test.

Such conditions may be expressed in simple or perfect tenses, in the active or passive voice, and in regular or progressive forms.

Conditions are of three basic kinds:

1.Possible: i.e., those in which both the condition and its fulfillment are possible.
Example: If I see him, I will tell him.

2.Contrary-to-fact: i.e., those in which the condition is contrary to the actual facts at the time spoken of and its fulfillment possible only hypothetically (in imagination).
Example: If I were rich, I’d buy a sports car. (But I’m not rich, so I’ll have to keep my old Toyota)

3.Doubtful or uncertain: i.e., those in which the condition and its fulfillment are possible, but are felt by the speaker to be unlikely or doubtful.
Example: If you should ever happen to be in New Your, you might reach me at this address.

These three basic kinds of condition are distinguished by the use of different verb forms. We will discuss No. 1 (the Possible) first, since it most closely resembles the common indicative mode.
 

POSSIBLE CONDITIONS

Present meaning in both clauses.

(Used to express a habitual state of general truth.)

Examples:

• If I am right, I deserve a reward.
• Provided that the weather is good, he usually walks to work.
• Unless you are with people you know, you don’t talk much.
• If he is offered a chocolate, he usually takes it.
• If I am reading, I don’t like to be disturbed.

Note: Present or present progressive tenses in either active or passive voices may be used in either clause.
 

Past meaning in both clauses

(Used to describe situations either temporary or habitual in the past.)

Examples:

• If I was right, I deserved a reward.
• Provided that the weather was good, he often walked to work.
• If that was his idea, he didn’t express it very well.
• If the suspect was in New York, as he claims, nobody saw him there.
• If he was trying to impress his girl friend, he wasn’t succeeding very well.

Note: Past or past progressive tenses in either active or passive voice may be used in either clause.

Future meaning in both clauses

(Used to indicate clear future possibility). Such future possibly is sometimes adequately expressed by using the simple present in both tenses.

Examples: If I win the prize, I get a free trip to Europe.

Usually, however, the main clause is in a future tense (simple future or future perfect) while only the conditional clause remains in the present tense.

Examples:

• If I see him, I will give him your message.
• Provided we get back in time, we will be glad to come to your party. (main clause future)
• If he dies tomorrow, he will still have had a full and happy life.  (Main clause future perfect)
   

Note 1: Active or passive forms may be used in both clauses, and progressives may also be used, under the same rules that govern the regular forms (see examples above).
Note 2: Although it is possible to use future or future perfect tenses in the conditional clause, such constructions are not idiomatic, and their use is not recommended.
 

Present perfect meaning in one or both clauses.

Examples:

• If I have been unjust to you, I am sorry.  (Perfect conditions: Present fulfillment)
• If this project has been carefully planned, it will probably be successful. (Perfect condition; future fulfillment)
• If I have worked hard, I have only done my duty. (Perfect in both clauses)
• If he has been lying to me, I have been wasting my time trying to help him. (Perfect progressive in both clauses.)
   

Note: Present perfect conditional sentences, like other conditionals, permit use of active and passive voiced, and regular or progressive forms.

 

CONTRARY-TO-FACT CONDITIONS

Present meaning.

Examples:
If I were you, I wouldn’t do that. (But, of course, I’m not you, so you must decide).
If I had a four day holiday, I would fly to Europe. (But I have only a two day holiday, so I can’t).

Note: Notice that past forms are used for present meanings in these constructions in order to suggest the unreality of the situations described.  Past forms used to suggest unreal, uncertain, or doubtful present situations are actually examples of the past tense of the subjunctive mode.  This mode is very important in some languages but is passing out of use in English and is being replaced by the indicative mode in all but a few grammatical situations.   Except in the case of the verb “BE”, the past subjunctive is identical with the simple past.

Example:
If I were you……
If you were her…..
If he were here…….
If we were there……
If they were……..

Notice that were is used for all persons.
 

Past meaning. (Active or Passive)

Examples:

• If I had had a car then, I would have driven, instead of going by plane.  (But at the time I didn’t have a car.)
• If we had been living in San Antonio at that time, we would probably have met the Colonel. (But we were living in California then.)
• If I’d known you were coming, I’d have bought more food. (But I didn’t, so we have nothing special to eat.)
• If the ship had been given advance warning, it would have been better prepared to meet the storm.
• Unless we had been walking by the river, we would never have heard the child cry for help.  (But we were and we did.)
• If I had seen him, I would have given him the message. (But unfortunately, I didn’t see him.)

Note 1: Notice that since we use simple past forms to represent unreal condition, we must, if we wish to represent a past unreal condition, go a step further back in grammatical time.   Thus in past contrary-to-fact conditions we employ the past perfect in the subordinate clause, and the past modals would, could, might, should, etc.  + have + past participle of the verb in the main clause.  If we wish to use progressive forms (there are several examples above), we use the past perfect progressive in the conditional

Note 2: In English the use of past perfect forms in the conditional sentences regularly implies that the events described were possible only in imagination and did not actually take place.
 

DOUBTFUL CONDITIONS (Active and Passive)

One further variety of condition remains to be mentioned. It expresses doubt, uncertainty or hesitation concerning something future. Structurally, it follows the patterns of contrary-to-fact present but in intention and significance it is perhaps closer to group ”present possibility”. We can understand one of its important uses more easily if we remember the polite use of would and could, etc. to soften commands and requests.

Example:

• Please open the window.
• Would you mind opening the window? (I’m not sure that you won’t mind.)
• Can I have another cigarette?
• Could I have another cigarette, please? (Perhaps I shouldn’t ask this.)

Similarly, doubtful conditional forms are sometimes used to soften requests for some future action that the speaker hesitates to demand more openly.

Examples:

• If you would help me lift this trunk, I would be very grateful. (Please help me lift it.)
• If you should happen to see Rafael, would you give him this letter? (asking a favor)
• If you’d study more, you’d get better grades. (I want you to study more, but I can’t compel you to.)

 Another typical use of Doubtful conditional forms is their employment with the stressed word only to express strong desire for some event that seems uncertain or unlikely to occur.

Examples:

• If it would only stop raining, we could go to the drive-in- tonight. (But it looks as though it may rain for a week!)
• If we would only follow my advice, he could finish that work in a week. (But you know how Joe is; he always likes to do things his way!)
•  A third very important use of Doubtful conditional forms is to state a future condition that is hypothetical but not contrary to possibility or even very unlikely.

Examples:

• What would you do if he refused to go? (He may, you know.)
• If this information ever became public, it could do a great deal of damage to whichever party was in power at that time. (I’m afraid this may happen.)
• If I should happen to be in New York at the same time he is, I would be glad to give him your message.
• If you could meet me tomorrow night at 8 o’clock in front of the Theater, we could go to the last show together. (Weighing possibilities or future action.)

Progressive forms of the Doubtful condition are the same as those of “Present contrary-to-fact.”

Most of the uses of Conditional forms which the student will meet in conversation or in his/her reading can probably be grouped under one of the main headings mentioned above. However, within these broad groupings the examples he/she will encounter may vary considerably from the norms given here, particularly in the time relationship between the condition and its fulfillment.
It will help if the student keeps a record of all conditional forms that give him trouble.  After he/she has mastered their meaning, he/she should practice writing new sentences that contain similar relationships of time and possibility. In this way, he/she will eventually acquire skill in a difficult area of English Grammar.