When and while are two very useful conjunctions in English that serve to introduce subordinate clauses related to the moment in which an action takes place. Both allow us to indicate that an action occurs at the same time as another, and they are sometimes interchangeable.

• We usually use when for single actions that happen almost simultaneously or very close together (one after the other).

Ex: He was walking home when he heard the explosion.
When my mother was working, I would do my homework on my own.

• We usually use while when we have two continuous actions taking place at the same time.

Ex: He was walking home while the thieves prepared the heist. 
While he was cleaning, he whistled a merry tune.

EXERCISES

1.  I saw him when / while I was on my way home
2. When / While he was young, he used to smoke a lot
3. George had a terrible accident while / when he was a child
4. When / While Jane was taking a bath, the phone rang three times
5. When / While we lived in London, we met many interesting people
6. Where are my friend while / when I most need them?
7. While / When I was driving, I listened to a very onteresting radio program
8. Who did you meet when / while you arrived to the party?
9. When / While I was speaking, they were thinking of something else!
10. I had many problems when / while I was a teenager