BY: Cristian yumisaca

USES OF

BY THE TIME
By the time is followed by a clause that (1) relates a time-frame, for viewing the progress or completion of the activity in the main clause; (2) includes an activity with duration and an end-point—possibly before and up to X time.
FUTURE PROGRESSIVE
The sun will be setting by the time I get home.
FUTURE PERFECT
The sun will have set by the time I get home.

WHEN
When is followed by a clause that (1) relates a point in time for viewing the progress or completion of the activity in the main clause; (2) includes an acitivity with relatively short duration or its duration is of little importance—at X time.
FUTURE PROGRESSIVE
The sun will be setting when I get home.
FUTURE PERFECT
~The sun will have set when I get home.