IF vs WHEN

if or when When we are using if or when in the sense of whenever, in other words to talk about repeated predictable actions, it doesn't matter which we use as there is very little difference in meaning. Compare the following: When I'm broke, I borrow money from my mother. I borrow money from my mother if I'm hard-up. Whenever I'm short of cash, I borrow money from my mother. I always feel sick if I go for a ride in your car. I always feel nauseous when I go for a ride in your car. I always feel queasy whenever I go for a ride in your car. when not if If it is certain that something has happened, is happening or will happen, we have to use when: When I arrived back home, Sandra was no longer there. You can have a drink when we get to Julie's place. Not now, when you're driving. if not when If it is uncertain that something has happened or will happen, we have to use if: He'll be here soon, if he caught the 1.30 train. If I manage to get to Argentina next year, I'll come and visit you. It is in this sense that the meaning corresponds to in the eventthat or in case of. We sometimes use these expressions instead of if, if we wish to sound more formal. Compare the following: In the event that you are unable to fly on 1 May, I would advise you to buy a travel ticket with greater flexibility. What happens if you can't fly on 1 May? Don't buy a travel ticket which limits your options. In case of frost, the race meeting will be cancelled. If frost is forecast, the race meeting will be called off.

			if or whether Restrictions also operate in the use of if and whether. If andwhether, of course, always imply uncertainty. You can generally use if or whether when you are reporting yes/no questions. It doesn't matter which you choose: Can you take care of Tommy for me? ~ I don't know yet whether / if I shall be able to look after Tommy during the Easter holidays or not. Is Paco still around? I wondered whether / if Paco had left the country as I hadn't seen him for some time. whether not if However, after prepositions, before to-infintives, inwhether…or… constructions and whenever we start a sentencewith the conjuntion, we have to use whether: Whether I'll get there in time for Henry's lecture, I don't know. I can't make up my mind whether to buy some new summer clothes now or wait until the prices go down.. There was a lot of discussion about whether Ringo should have his tongue pierced or not. Whether we go by bus or by train doesn't really matter. It's bound to be a slow journey. I'm going to spend all of granny's money on a new kitchenwhether you like it or not!

			if not when in 'conditional' sentences In 'conditional' sentences, we normally use if. If we are describing something which must happen first so that something else can occur, i.e. referring to a condition for something to happen, we have to use if, unless if means whenever. Consider these three conditional sentences below and then scroll back to the very first examples to remind yourself about if meaning whenever: I'll help you landscape the garden, if I can get off work for a couple of weeks. Don't take any chances. What would you do if you lost your job? If my dad had been alive, he would've sorted it all out for you in a matter of days.

From Different sources

When or if? 
We use when for things that are certain to happen in the future: 

I’ll buy you a pair of gloves when I go shopping this afternoon. 
I’ll give you a ring when I get home from school. 
Note that although the reference is clearly to the future, we use the present tense in the when-clause. 

We use if for things that may happen and which we are not certain about: 

If I decide to come to London this year, I’ll come and visit you. 

I’m going to call the police if she’s not back within the hour. 

If you make the salads, I’ll prepare the barbecue. 
In all of these examples, we are talking about future conditions and whether we use when or if depends upon the certainty of things happening. 

However, when and if are interchangeable when we are talking about general conditions that always apply when if means almost the same as whenever. Compare the following: 

I keep the air-conditioning on at night if the temperature goes above 30 degrees. 

I keep the air-conditioning on at night whenever / when the temperature goes above 30 degrees. 

If the green flag is flying, it’s quite safe to swim here 
You can swim here whenever / when / provided the green flag is flying. 

if vs. when

if and when are interchangeable when the statement of the conditional clause is a fact or a general issue (also known as zero conditonal)
If you heat ice, it melts.
When you heat ice, it melts.

if is used for something that, according to the speaker, might happen.
We can spend the afternoon on the beach if the weather is fine.

when is used for something that, according to the speaker, will happen.
I will clean up the kitchen right away when I'm back from work.

in case vs. if

in case of can be used to shorten an if-clause as shown below:
If there is a fire, leave the room.     In case of fire, leave the room.

While if expresses a condition (1), in case is used to express a possibility (2).
(1)     I need painkillers if I'm in severe pain.
(2)     I need painkillers in case I'm in severe pain.

The expression just in case is used pretty much the same way.
I got you a pizza just in case you were hungry.
(I don't know whether you are hungry.)


Example: 
Represented by widening income and standard of living gaps, increasing economic disparity, when exacerbated simultaneously by population growth, often leads to social unrest and rioting.

A. when exacerbated simultaneously by population growth, often leads
B. when exacerbated by simultaneously grown population, often lead
C. when being exacerbated by simultaneous growth in population, often leads
D. if exacerbated simultaneously by population growth, often lead
E. if exacerbated by simultaneous population growth, often leads