Complete the sentences with a subordinator: although, so that, because, when

I. Complete the sentences with a subordinator : although , so that , because , when . 
1. He didn't go to school ....... it rained heavily . 
2. She walked home alone at night ......... she knew that it was dangerous . 
3. ........ he has free time , he often goes to the park . 
4. She is studying hard ..... she can pass the exam .