by and by vs then

by and by

• After a short time. 

• After an indefinite period. 

• Heaven; the hereafter. Usually preceded with "the sweet." 

then

• Soon afterward. 

• Next in order of place. 

• At that time. 

• In addition; also; besides. 

• At the same time; on the other hand. 

• Used to contradict an assertion. 

• In that case. 

• Being so at that time. 

• That time