He comes back.I'll tell you at once.改为条件状语从句 格式：I ___ ___ you at once ___ he comes back
问题描述：
He comes back.I'll tell you at once.改为条件状语从句 格式：I ___ ___ you at once ___ he comes back
答
will tell if
条件状语从句，主句将来时，从句一般选择时
答
will tell if
答
will tell when