and that there will come a time when they won't be feeling s

and that there will come a time when they won't be feeling so good.
When you observe peaceful,relaxed people,you find that when they are feeling good,they are very grateful.They understand that both positive andnegative feelings come and go,「and that there will come a time when they won't　be feeling so good.」最后一句的 won't　be feeling so good,语法请讲解一下,