Conditional Sentences

There are many different ways of making sentences with "if". It is important to understand the difference between sentences that express real possibilities, and those that express unreal situations.

• Real possibilities: Ex: If it rains, we will stay at home.
• Unreal situations: Ex: if I were rich, I wouldn't have any problems.

Real Conditional Sentences - Present

a. Form: IF + S + Vinf, S + WILL + Vinf (without to)
Ex: if I work hard, I will pass my exam. If she has enough money, she will buy a new car. If we don't hurry up, we will be late. If you are late, I won't wait for you. What will you do if you don't go to university?
Note: The condition clause "if" can come at the beginning of the sentence or at the end. If it comes at the beginning, we put a comma at the end of the clause. If it comes at the end, we do not use a comma. Ex: if I work hard, I will pass my exam. Will you go to university if you pass your exam?
b. Uses:
The first conditional is used to express a possible condition and a probable result in the future. Ex: if my cheque comes, I will buy us all a meal. You will get wet if you don't take an umbrella. What will happen to the environment if we don't look after it?
We can use the first conditional to express different functions (all of which express a possible condition and a probable result). Ex: if you do that again, I will kill you! (=A threat). Careful! If you touch that, you will burn yourself. (= A warning). I will post the letter if you like. (= An offer).
Note:

• English uses a present tense in the condition clause, not a future form. Ex: if it rains... (Not: if it will rain). If she works hard... (Not: if she will work hard).
• "If" expresses a possibility that something will happen, "when" expresses what the speaker sees as certain to happen. Ex: if I find your book, I will send it to you. When I get home, I will have a bath.

Unreal Conditional Sentences

a. Form: IF + S + Ved, S + WOULD + Vinf (without to)
"Would" is a modal auxiliary verb. The forms of would are the same for all persons. Ex: if I had more money, I would buy a new house. If we lived in Russia, we would soon learn Russian. What would you do if you had a year off? Would you travel around the world?
Note: "were" is often used instead of "was" in the condition clause. If I were you, I would go to bed early. If he were cleverer, he would know he was making a mistake.
b. Uses:
The second conditional is used to express and unreal or improbable condition and its probable result in the present or future. The condition is unreal because it is different from the facts that we know. We can always say "but... Ex: if I were Prime Minister, I would increase tax for rich people. If I lived in a big house, I would have a party. What would you do if you saw a ghost?
Note:
The use of the past tense (if I had) and "would" does not refer to past time. Both the first and second conditional refers to the present and the future. The past verb forms are used to show "this is different from reality". Ex: if I will the tennis match, I will be happy. If I won a thousand pounds, I would...
We do not use "would" in the condition clause. Ex: if the weather was nice... (Not: if the weather would be nice...) If I had more money... (Not: if I would have more money).
Note: first or second conditional?
Both conditionals refer to the present and future. The difference is about probability, not time. It is usually clear which conditional to use first conditional sentences are real and possible; second conditional sentences express situations that will probably never happen.
Ex: if I lose my job, I will... (My job is doing badly. There is a strong possibility of being made redundant.)
Ex: if I lost my job, I would... (Redundancy probably won't happen. I'm just speculating.)

Unreal Conditional Sentences - Past

a. Form: IF + S + HAD + P2, S + WOULD + HAVE + P2
b. Uses:
Third conditional sentences are not based on fact. They express a situation, which is contrary to reality in the past. This unreality is shown by a tense shift from past to past perfect. Ex: if you had come to the party, you would have had a great time. I wouldn't have met my friend if I hadn't gone to Vietnam.
It is possible for each of the clauses in a conditional sentence to have a different time reference, and the result is a mixed conditional. Ex: if we had brought a map (we didn't), we would know where we are (we don't).
Note: Zero conditional
Zero conditional sentences refer to "all time", not just the present or future. They express a situation that is always true. "If" means "when" or "whenever". Ex: if you spend over 20$ at that supermarket, you get a 5% discount.

Other ways of expressing a condition

If

• Used to say that one thing can, will or might happen or be true, depending on another thing happening or being true: If anyone calls, tell them I'm not at home. If he improved his IT skills, he'd (= he would) easily get a job. You would know what was going on if you'd (= you had) listened. They would have been here by now if they'd caught the early train. If I were in charge, I'd do things differently. (Rather formal) If I were in charge... Even if (= although) you did see someone, you can't be sure it was him.
• When; whenever; every time: If metal gets hot it expands. She glares at me if I go near her desk.
• (Formal) used with will or would to ask sb politely to do sth: If you will sit down for a few moments, I'll tell the manager you're here. If you would care to leave your name, we'll contact you as soon as possible.
• Used after ask, know, find out, wonder, etc. to introduce one of two or more possibilities. SYN whether: Do you know if he's married? I wonder if I should wear a coat or not. He couldn't tell if she was laughing or crying. Listen to the tune and see if you can remember the words.
• Used after verbs or adjectives expressing feelings: I am sorry if I disturbed you. I'd be grateful if you would keep it a secret. Do you mind if I turn the TV off?
• Used to admit that sth is possible, but to say that it is not very important: If she has any weakness, it is her Italian. So what if he was late. Who cares?
• Used before an adjective to introduce a contrast: He's a good driver, if a little over-confident. We'll only do it once - if at all.
• Used to ask sb to listen to your opinion: If you ask me, she's too scared to do it. If you think about it, those children must be at school by now. If you remember, Mary was always fond of animals.
• Used before could, may or might to suggest sth or to interrupt sb politely: If I may make a suggestion, perhaps we could begin a little earlier next week.

IDIOMS
If and when: Used to say sth about an event that may or may not happen: If and when we ever meet again I hope he remembers what I did for him.
If anything: Used to express an opinion about sth, or after a negative statement to suggest that the opposite is true: I'd say he was more like his father, if anything. She's not thin - if anything she's on the plump side.
If I were you: Used to give sb advice: If I were you I'd start looking for another job.
If not

• Used to introduce a different suggestion, after a sentence with if: I'll go if you're going. If not (= if you are not) I'd rather stay at home.
• Used after a yes/no question to say what will or should happen if the answer is "no": Are you ready? If not, I'm going without you. Do you want that cake? If not, I'll have it.
• Used to suggest that sth may be even larger, more important, etc. than was first stated: They cost thousands if not millions of pounds to build.

If only: Used to say that you wish sth was true or that sth had happened: If only I were rich. If only I knew her name. If only he'd remembered to send that letter. If only I had gone by taxi.
It's not as if: Used to say that sth that is happening is surprising: I'm surprised they've invited me to their wedding - it's not as if I know them well.
Only if: (Rather formal) used to state the only situation in which sth can happen: Only if a teacher has given permission is a student allowed to leave the room. Only if the red light comes on is there any danger to employees.
If / whether
Both if and whether are used in reporting questions which expect "yes" or "no" as the answer: She asked if / whether I wanted a drink. Although whether sounds more natural with particular verbs such as discuss, consider and decide. When a choice is offered between alternatives if or whether can be used: We didn't know if / whether we should write or phone. In this last type of sentence, whether is usually considered more formal and more suitable for written English.
Unless
Used to say that sth can only happen or be true in a particular situation: You won't get paid for time off unless you have a doctor's note I won't tell them - not unless you say I can. Unless I'm mistaken, she was back at work yesterday.
Used to give the only situation in which sth will not happen or be true: I sleep with the window open unless it's really cold. Unless something unexpected happens, I'll see you tomorrow.
Help Note: Unless is used to talk about a situation that could happen, or something that could be true, in the future. If you know that something has not happened or that sth is not true, use "if ... not": If you weren't always in such a hurry (= but you are), your work would be much better.
Whether
Used to express a doubt or choice between two possibilities: He seemed undecided whether to go or stay. It remains to be seen whether or not this idea can be put into practice. I asked him whether he had done it all himself or whether someone had helped him. I'll see whether she's at home (= or not at home). It's doubtful whether there'll be any seats left.
Used to show that sth is true in either of two cases: You are entitled to a free gift whether you accept our offer of insurance or not. I'm going whether you like it or not. Whether or not we're successful, we can be sure that we did our best.