用英文讲好了...
it should be 'Susan was unhappy that...'
"Susan was unhappy when her mother asked her to babysit her younger brother..."
Up to this point the sentence seems correct because it is describing Susan's state during the time that her mother asked her to babysit her brother.But when the sentence continues into "because Susan found his behavior irritating" we are not sure if this also occurred during the same time,or before it.The sentence means to say that Susan found his brother's behavior irritating before her mother's request (something she had developed - she had always found him to be annoying) as well as during it
The sentence uses simple past so it would be confusing to use 'when'
For example,"I was happy when I picked up the 20 dollar bill because I was poor" describes 3 things (picking up the bill,being happy,and being poor) that occurred at the same time.On the other hand,"I was happy that I picked up the dollar bill because I was poor" describes being poor before picking up the bill.