6.2.6 IF, EVEN IF, ONLY IF, AS LONG AS, PROVIDED, SUPPOSING, UNLESS, IF NECESSARY, IF SO etc.

IF and WHEN

When can replace if in zero conditionals:

If you heat water to 100 degrees Celsius, it boils.
When you heat water to 100 degrees Celsius, it boils.

In the other types of conditionals, we cannot use when instead of if.

EVEN IF

Even if emphasises that something will happen, would happen or would have happened whatever the condition:

Even if we leave right now, we still won't catch the train.
I wouldn't go into the water even if I could swim.
Even if we had booked our flight earlier, it wouldn't have been cheaper.

ONLY IF

Only if makes the condition more restrictive:

Acetaminophen is dangerous to children only if dosage is too high.

If the if-clause is first, the subject and the auxiliary in the main clause are inverted:

Only if you like classical music is it worth coming tonight.

SO/AS LONG AS, PROVIDING/PROVIDED (THAT)

So/As long as and providing/provided (that) can be used instead of if to express a condition. Note that providing/provided (that) is a bit formal:

You can stay here as long as you keep quiet.
Provided/Providing (that) the bills are paid, tenants will not be evicted.

SUPPOSE/SUPPOSING, WHAT IF

Suppose/supposing and what if can replace if, mainly in everyday conversation, and are often used without a main clause:

Suppose/supposing you won the lottery, what would you do?
Suppose/supposing you can't find a job?
What if you are not accepted to university? What will you do then?

UNLESS

Unless is followed by an affirmative verb to express "if ... not":

My leg's broken. I can't stand up unless you help me. (I can't stand up if you don't help me.)

IF IN DOUBT, IF POSSIBLE, IF NECESSARY etc.

We can make the if-clause shorter by omitting the subject and the verb be:

If (you are) in doubt, consult a dictionary.

In certain idiomatic expressions, the subject and be are normally omitted:

If necessary, you can call Jake at home.
I'd like a seat by the window if possible.

IF SO, IF NOT

If so and if not can stand for an if-clause which is understood from the context:

"According to the weather forecast, it might rain tomorrow." "If so, we'll go hiking another day."
I hope Peter gets here soon. If not, we'll have to start without him.