Theory:
Time elements in sentences are commonly tested in sentence completion exercises, and examples of these are:
for,
since,
until,
while,
just about to,
gone by,
by,
during,
by the time
before
The easiest way to understand how to use them is by looking at them in context:
for,
I will for you for twenty minutes.
since,
I have been waiting since six o'clock.
until,
I will wait until you call.
while,
I had a beer while I waited.
just about to,
I was just about to call.
gone by,
More than an hour had gone by when he called.
by,
I will get there by 9 o'clock.
during,
I met Sally during my summer holidays.
by the time
I will get there by the time you wake up.
before
I will call you before you get there.
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