if

Supposing that, assuming that, in the circumstances that; used to introduce a condition that may be (or prove to be) either true or false.

If crossing the road, make sure you look both ways.

If I give you the money would/will you buy it for me?

Supposing that, assuming that, in the circumstances that; used to introduce a condition that may be (or prove to be) either true or false.

if A then B else C

Supposing that; used with past or past perfect subjunctive to indicate a counterfactual or hypothetical condition.

If she hadn't told me, I wouldn't know.

"You would be healthier if you would give up smoking" "Well, if I were rich, I'd go to one of those hypnosis therapies to quit".

Supposing that; used with past or past perfect subjunctive to indicate a counterfactual or hypothetical condition.

I wouldn't marry you if you were the last man on earth.

You're finishing your chowder if you sit there all afernoon!

Considering the fact that; given that; introducing a condition that is known to be true.

The drain's blocked — and if the drain's blocked, the water won't flow.

O what of Gods then boots it to be borne, / If old Aveugles ſonnes ſo euill heare?

An uncertainty, possibility, condition, doubt etc.

The board can't approve this project; there are too many ifs.

1709, Susannah Centlivre, The Busy Body, Act III, in John Bell (ed.), British Theater, J. Bell (1791), page 59, Sir Fran. Nay, but Chargy, if——— ¶ Miran. Nay, Gardy, no Ifs.——Have I refus'd three northern lords, two British peers, and half a score knights, to have put in your Ifs?

Initialism of interactive fiction.

The “Zarfian Cruelty Scale” rates games as Merciful, Polite, Tough, Nasty, or Cruel. The scale describes how works of IF become unwinnable, especially how and when the interactor (here a player, and one trying to win) learns this.

Initialism of impact factor.

Initialism of intermediate filament.

Initialism of intermediate frequency.

Initialism of intermittent fasting.