My brother was fishing when a big wave broke on him.
We were sailing when we saw some dolphins!
We were having lunch when some seagulls took our sandwiches.
A big wave broke on him while my brother was fishing.
We saw some dolphins while we were sailing.
Some seagulls took our sandwiches while we were having lunch.
Was / Were + v (ing) WHEN verb in 2nd Column.
Verb in 2nd Column WHILE was/were + v (ing)