If something happens when something else is happening, the two things are happening at the same time. When eating a whole cooked fish, you should never turn it over to get at the flesh on the other side Mustard is grown in the field when weeds are there, rather than when the growing crops are there
At what time; at, during, or after the time that; at or just after, the moment that; used relatively
You use when to introduce the reason for an opinion, comment, or question. How can I love myself when I look like this?
You use when in order to introduce a fact or comment which makes the other part of the sentence rather surprising or unlikely. Our mothers sat us down to read and paint, when all we really wanted to do was to make a mess = although