Conditional sentences

A. Zero conditional
IF     +    DOES,         DOES

If + present simple tense,  present simple tense

1. If is used to state general rules.
    If we heat water enough, it begins to boil.
In statements like this, if means the same as when or every time.

B. The 1st conditional

IF    +    DOES,         WILL DO

If + present simple tense,   will + bare infinitive

2. In 1st conditional if 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.
    If they offer a good price, we will buy the whole consignment.
COMMON MISTAKE. We do not use will in the if part of the sentence.

3. When we talk about an event that will take place in the future, we can use if or when.

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.)
I am flying to the States tonight. I'll give you a ring when I get there.
(The speaker has no doubt that the plane will arrive safely.)
4. 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.
Note. We say the traffic but a traffic jam
5. 1st conditional is usually used in such cases: C. Conditionals: if, unless, in case, provided that, as long as, so that

6. If and unless

Unless means the same as if ... not. It always refer 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.

7. 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 afterward.)

If the goods get damaged in transit, we'll make a claim.
(The damage may happen, and we will make a claim afterward.)

Note that that in sentence with in case, we often use going to rather than will because we are often talking about something that we have already decided to do.

8. Provided that 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.)
9. 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)

D. 2nd conditional

IF     +    DID,         WOULD DO

If + past tense,  would  +  infinitive

10. 2nd conditional can be used to refer to less probable or impossible situations. The verb in the second part is preceded by would / should / could / might.
The if-clause can come in the first part of the sentence, or the second.
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.

11. This form refers to present or future time.

If these machines were not so expensive, we would buy them.
If we hired a lawyer, we would recover our debts more 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. 2nd conditional is usually used in such cases

13. First or second conditional

If we think that future event is reasonably likely, we use first conditional

If the market grows at 7% a month, it will involve new investment rapidly.
If we are talking about an event that is unlikely or impossible, we use the second conditional
If I had as much money as Bill Gates of Microsoft, I would retire.
14. Variation

It is also possible to use might and could instead of would

If we received credit, we could expand much more rapidly.
In the if-clause , we can use were instead of was. This is very common when we give advice using the expression If I were you ...
If I were you, I would have another look through those figures.

E. 3rd conditional

IF    +    HAD DONE,    WOULD     +    HAVE DONE

If + past perfect tense, would  +  present perfect

15. 3rd conditional is used when talking about things that didn't happened 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). If I'd known it was formal party, I wouldn't have gone wearing jeans and jumper. I would have worn suit. 16. 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.
If I had not been in Amsterdam at the last RIPE meeting, I would not have met Esther Dyson and I wouldn't have known she speaks Russian.
17. Mixed conditionals

If we talk about a  past action and its result in the present we use if + past perfect and would not + infinitive:

If he hadn't done well on the training courses, he wouldn't be a Project leader now.

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1999. Yuri Demchenko.