Definition of if conjunction from the Oxford Advanced Learner's Dictionary

Oxford3000

if

conjunction
ɪf
 
; ɪf
 
 
1 used to say that one thing can, will or might happen or be true, depending on another thing happening or being trueIf you see him, give him this note.I'll only stay if you offer me more money.If necessary I can come at once.You can stay for the weekend if you like.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 was 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.2 when; whenever; every timeIf metal gets hot it expands.She glares at me if I go near her desk.3 (formal) used with will or would to ask somebody politely to do somethingIf 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.4 used after ask, know, find out, wonder, etc. to introduce one of two or more possibilities
Synonym
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.
5 used after verbs or adjectives expressing feelingsI 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?6 used to admit that something is possible, but to say that it is not very importantIf she has any weakness, it is her Italian.So what if he was late. Who cares?7 used before an adjective to introduce a contrastHe's a good driver, if a little over-confident.We'll only do it once— if at all.8 used to ask somebody to listen to your opinionIf 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.9 used before could, may or might to suggest something or to interrupt somebody politelyIf I may make a suggestion, perhaps we could begin a little earlier next week.
Idioms

if and when

used to say something 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 something, 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 somebody advice
If I were you I'd start looking for another job.

if not

1 used to introduce a different suggestion, after a sentence with ifI'll go if you're going. If not (= if you are not) I'd rather stay at home.2 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.3 used to suggest that something may be even larger, more important, etc. than was first statedThey cost thousands if not millions of pounds to build.

if only

used to say that you wish something was true or that something 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 something 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 something can happenOnly 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.
Usage noteUsage note: if / whetherBoth 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.