Task: Pos Tagging
Input (English): Sentence: Henrietta , however , was at that time engaged in a lengthy correspondence with Joe's older and more serious brother , Morris , who was just about her own age and whom she had got to know well during trips to Philadelphia with Papa , {{ when }} he substituted for Rabbi Jastrow at Rodeph Shalom Temple there during its Rabbi's absence in Europe . 
Word: when
Output (English): ADV