1. CONDITIONAL SENTENCES

2. STRUCTURE

3. In grammar,conditional sentences are sentences discussing factual implications or hypothetical implications and their consequences. Languages use a variety of conditional constructions and verb forms (such as the conditional mood)to form this kind of sentences.
Full conditional sentences contain two clauses, the condition or protasis and the consequence or apodosis.
“If it rains [condition], (then) the picnic will be cancelled [consequence]”.
Syntactically, the result is the main clause, and the condition is a subordinate clause.

4. TYPES

5. ZERO CONDITIONAL :
PROBABLE CONDITON

6. STRUCTURE: If + present simple tense, present simple tense
zero conditional sentence consists of two clauses, an "if" clause and a main clause
STRUCTURE: If + present simple tense,  present simple tense
They are used to state general rules. To generalize or give facts. To talk about something which is temporarily true or always true
“If you heat water to 100 degrees, it boils”.
If the "if" clause comes first, a comma is usually used. If the "if" clause comes second, there is no need for a comma:
“Water boils if you heat it to 100 degrees”.
NOTE: Other tenses as present continuous can be used.
“If your muscle hurts, you are probably doing to much exercise”.

7. FIRST CONDITIONAL:
PROBABLE CONDITION

8. The 1st conditional is used to speculate about the future consequences of a specific event. In this case, the verb in the second part of the sentence is preceded by will.
GENERAL STRUCTURE: If + present simple tense,   will + bare infinitive
“If they offer a good price, we will buy the whole consignment”.
You can use any present tense in the if-clause, and any future tense in the other clause.
“I´ll have finished in an hour if you don´t disturbe me”
“If you are not going, I´m not going to go either”.
COMMON MISTAKE: We do not use the future in the if part of the sentence.
“I am flying to the States tonight. I'll give you a ring if I can find a phone”. (The speaker is not sure if he will be able to find a phone or not.) In a sentence with an if-clause we can use the imperative, or other modal verbs, instead of will + infinitive
“If you hear from Susan today, tell her to ring me.” “If the traffic is bad, I may get home late”.

9. The 1st conditional is usually used in the following cases:
Contingency plans, considering events that may/may not occur (If I feel too excited to sleep, I'll try reading one of our reports)
Company forecast; degrees of certainty; project planning,…

10. SECOND CONDITIONAL:
IMPROBABLE CONDITION

11. STRUCTURE: If + past tense, would + infinitive
The 2nd conditional can be used to refer to less probable or impossible situations. The verb in the second part is preceeded by would / should / could / might. The if-clause can come in the first part of the sentence, or the second.
STRUCTURE: If + past tense,  would  +  infinitive
“If I knew her number, I would send her a fax/I would send her a fax if I knew her number”.
COMMON MISTAKE: We do not use would in the if part of the sentence.
“If these machines were not so expensive, we would buy them. / If we hired a lawyer, we would recover our debts easily/If I lost my job tomorrow, I would move to London to find the same kind of job.”
The first two sentences refer to present situation, and imagining a situation that is different from the reality. In the third we are talking about a possible event in the future, but using second conditional we make it clear that we do not really think it will happen.

12. The 2nd conditional is usually used in the following cases:
Stating preferences
Supposing : “If I were 10 years younger, I'd take the job”.
Unusual circumstances: “I would / might join the army, if there was a war”.
NOTE: If we think that a future event is reasonably likely, we use first conditional. If we are talking about an event that is unlikely or impossible, we use the second conditional

13. THIRD CONDITIONAL:
IMPOSSIBLE CONDITION

14. STRUCTURE: If + past perfect tense, would + present perfect
The 3rd conditional is used when talking about things that didn't happen in the past (and the consequence if they had happened). The verb in the second part is used with would / should / could / might (+ have + past participle).
STRUCTURE: If + past perfect tense, would  +  present perfect
IF    +    HAD DONE,    WOULD     +    HAVE DONE
“If I'd known it was a formal party, I wouldn't have gone wearing jeans and a jumper. I would have worn a suit.”
Positive and negative: When we use the 3rd conditional we are imagining the opposite situation. If what actually happened was negative, we use a positive form. If what actually happened was positive, we use a negative form:
“If my client had given me her fax number, I wouldn't have had to post a letter to her”.
“If I'd known it was a formal party, I wouldn't have gone wearing jeans and a jumper”.

15. MIXED CONDITIONALS
Unreal conditionals (type II + III) sometimes can be mixed, that is, the time of the if clause is different from the one of the main clause.
Past --> Present: If I had taken an aspirin, I wouldn't have a headache now.
Past --> Future: If I had known that you are going to come by tomorrow, I would be in then.
Present --> Past: If she had enough money, she could have done this trip to Hawaii.
Present --> Future: If I were you, I would be spending my vacation in Seattle.
Future --> Past: If I weren't flying to Detroit, I would have planned a trip to Vancouver.
Future --> Present: If I were taking this exam next week, I would be high-strung.

16. VARIATIONS

17. VARIATIONS
UNLESS means the same as if ... not. It always refers to the conditional part of the sentence and not the result part of the sentence:
“If he doesn't get here soon, we will have to start the meeting without him.” “Unless he gets here soon, we will have to start the meeting without him”.
We often use not + unless, which means only ... if, when we want to emphasize a condition:
“They will only sign the contract if we give them an additional discount / They won't sign the contract unless we give them an additional discount.”

18. IF and IN CASE: We use in case to talk about precautions we will take before a problem happens. We use if to talk about what we will do after a problem happens:
“We are going to insure the shipment in case the goods get damaged in transit”. (We will take our insurance first; the problem may or may not happen afterwards.)
“If the goods get damaged in transit, we'll make a claim”. (The damage may happen, and we will make a claim afterward.)
Note: Do not confuse it with in case of, that you sometimes see in notices. E.g. In case of fire, break glass.

19. MORE VARIATIONS

20. PROVIDED vs AS LONG AS, etc.
We can use provided that/providing, as long as, and so long as when we want to emphasize condition. Provided that and as long as mean if and only if (providing and so long as are a little less formal):
“I will agree to these conditions provided that they increase my salary”. (I will only agree if they give me more money.)
“The strike will be successful as long as we all stay together”. (It will only succeed if we all stay together.)
Other similar variations are: assuming (that); on condition (that); on the assumption (that); supposing (that);with the condition (that).
SO THAT: We use so that to say what the result or purpose of an action will be:
“I'll take a credit card so that we don't run out of money”. (The credit card will stop us from running out of money)

21. OTHER CASES
EVEN IF describes how sth. will happen whatever the condition
“Even if it rains, we´ll still go for a picnic”.
SHOULD/WERE TO make an event seem more unlikely and hypothetical:
“If you should see Ann, could you ask her to call me?” (It implies that I do not expect to see Ann)
“If I were to ask you to marry me, what would you say?”
HAPPEN TO emphasises chance posibilities
“If you happen to see Mark, could you tell him to give me a call?”
IF IT WERE NOT FOR/IF IT HADN´T BEEN FOR describe how one event depends on another
“If it weren´t for Mary, this house would be a mess”

22. WILL/WOULD: politeness and emphasis
“If you will/would wait here, I´ll see if Mr Morgan is free”.
“If you will stay out late, no wonder you are tired!” (insist on staying out)
BUT FOR can be replaced by if not. It´s used in formal language, and must be followed by a noun form.
“But for your help, we wouldn´t have been able to do it!”
(If you hadn´t helped us, we wouldn´t have been able to do it!).
IF AND ADJECTIVES: In expressions such as if is necessary/possible, it is possible to omit the verb be.
“If interested, apply within / If necessary, take a taxi”.

23. OMITTING IF Inversion is used when “if” is omitted:
Had I known... (instead of: If I had known...)
Were you my daughter,... (instead of: If you were my daughter,...)
Should you need my advice,... (instead of: If you should need my advice,...)

24. PRACTICE http://www.englishpage.com/conditional/conditionalintro.html

25. FUTURE TIME CLAUSES

26. TIME EXPRESSIONS
As soon as
When
Until
Unless
Before
After
In case

27. After these time expressions, even though you are talking about the future, you need to use a present tense (simple, continuous, perfect)
We´ll be able to leave as soon as I lock the front door.
I´ll ring you when I get there.
I´ll be back before you know.