An uncertainty, possibility, condition, doubt etc.

Supposing that, assuming that, in the circumstances that; used to introduce a condition or choice.

“If it rains, I shall get wet.”

In the event that a statement is true (a programming statement that acts in a similar manner).

“If A, then B, else C.”

Supposing that; used with past or past perfect subjunctive indicating that the condition is closed.

“I would be unhappy if you had not talked with me yesterday.”