Study materials
In Ukrainian, when and while are the same word. So learners tend to pick one at random and hit the target about half the time.
English hears a clear difference between them. One word brings a short event with it, the other brings a long process. Once you hear that, the choice stops being a lottery.
I was sleeping when the phone rang.
While I was cooking, Ben set the table.
When brings the event, while brings the process
After when comes what happened in an instant: a call, someone walking in, a sudden sight. That is the Past Simple.
She was reading when I came in.
We were eating when the lights went out.
After while comes what was running: cooking, waiting, driving. That is the Past Continuous.
The lights went out while we were eating.
He called while I was driving.
Notice that the last two describe the same situations as the ones above, just from the other side. The events do not change; what changes is where the sentence starts.
The halves can swap places
Both orders are correct and mean the same. The only difference is the comma: if the sentence opens with the conjunction, that half needs one after it.
While we were waiting, it started to rain.
It started to rain while we were waiting.
When both actions are long
Not every sentence needs two different tenses. If both actions were running, both stay in the Continuous, with while between them.
She was reading while I was working.
The kids were playing while we were talking.
Which one is the background
This is where the real skill starts. The same two actions can be framed either way, and the listener hears two different stories.
I was leaving the house when it started to rain.
When I left the house, it started to rain.
In the first sentence leaving is the background; in the second it is an event. So the question is never "which conjunction" but "which of these was running".
The mistakes everyone makes
The first is putting while in front of a short event.
While the phone rang, I was sleeping. → When the phone rang, I was sleeping.
The second is putting both actions in the Past Simple and changing the meaning by accident.
I slept when the phone rang. → I was sleeping when the phone rang.
The third is dropping the comma when the sentence opens with the conjunction.
While we were waiting it started to rain. → While we were waiting, it started to rain.
All three are caught by one question. Which of the two actions is longer? The long one takes while and the Continuous; the short one takes when and the Simple.
Try it right now
Think of some small thing from the past week: something that interrupted you in the middle of a task.
Say it in English twice: start with the process, then start with the event. Listen to how the weight of the story moves.