If vs. Whether

If is used in conditional statements describing “if/then” situations in which one event follows from another:

The Spurs will win the championship if their best players are all healthy.

conditional: If their best players are healthy, then they will win.

Traditionally, whether, not if, should be used to refer to two options or possibilities:

not sure if they would win = two possibilities: winning and losing

not sure whether they would win

In informal English, it is quite common and generally accepted to use if in place of whether in cases like this, but in more formal contexts, remember to follow this rule.