When as an interrogative pronoun
Quite simply, when is used to ask questions related to time. For example,
When are you planning to come?
When will Tony reach London?
When was the last time you met your parents?
Contraction of “when is”
When is = when’s
§ When’s the next flight?
§ When’s your birthday?
When in statements and sentences
Any statement referring to time, can have when. Here when is not interrogative pronoun, rather it acts as a conjunction –
§ When I was a child, I used to play with dolls.
§ You should remain silent when the teacher enters the room.
§ We were having our dinner when my grandmother called.
§ When it stops pouring, we can go to the park.
Notice that the sentence can start with when and it would mean the same even if the placement of when is changed –
When I was a child, I used to play with dolls è I used to play with dolls when I was a child.
When in a sentence with future meaning
In sentences which refer to future, the clause after when is in present tense and not in future tense.
§ I will be in school when you reach home. (and not when you will reach home)
§ When I have more time, I will go for shopping. (and not when I will have time)
When as an adverb
As an adverb also, when can be used to ask questions. But, under a different context. ‘When’ need not necessarily mean time frame, but can mean ‘under what circumstances’ or ‘conditions’.
§ When should you say ‘Sorry’ to someone?
§ When are we supposed to make payments?
These questions can be changed to statements too –
He did not specify when we should say ‘Sorry’ to someone.
It is mandatory to know when to make the payments.
To choose between two alternatives
I don’t know why she had to play when she was not well.
The custard tastes best when taken chilled.
3D cinema gives the best experience when seen with glasses.
Notice the use of 3rd form (past participle form) of verb in these types of sentences.
Whatever the usage may be, there is still reference of time in some or the other way in all the above sentences. ‘When’ should always be used with respect to time.
To signify two simultaneous actions
Consider the following sentence –
“She watched TV when I was away.”
The above sentence can also be written as “She was watching TV while I was away”
There is a subtle difference both these statements. The first one tries to convey that ‘she’ watched tv for a particular time period, whereas the second one indicates two actions happening at the same time.
When I ride a bike, I always wear helmet. è I always wear helmet while riding a bike.
It is better to use while in sentences that talk about two simultaneous actions instead of using when.
§ While in Rome, be a Roman. Here, it would be wrong to say When in Rome, be a Roman.
§ I cooked food while she was sleeping – here we can use when too but while is more appropriate when there are two long actions taking place at the same time.
1. Use when for situations related to time.
2. Use while for describing two actions happening at the same time.