during

We use during to talk about something that happens at one point within a period of time or to talk about an event that continues throughout a whole period of time. Compare the following:

• I sometimes wake up during the night and then I can't go back to sleep again.
  I cried during the performance. It was such a sad play.
  
• During the school holiday period in the summer all the campsites are full.
  During wars food is often rationed.

When we are referring to a whole period of time, we sometimes use throughout as an alternative to during for emphasis:

• Sugar and cheese continued to be rationed throughout the post war period.
• These hotels are usually fully booked throughout the summer season.

We sometimes use in as an alternative to during to talk about something that happens within a particular period of time:

• I sometimes wake up in the night and can't get back to sleep again.
• In my fours years as head of this company I have only taken a holiday once.

If the activity continues for a period of time, we sometimes use over instead of during to describe the specified period:

• Over the last few days, weather conditions have been steadily improving and a rescue now seems possible
• I don't intend to do very much over the summer - just relax!

for

During tells us about the period when something happens. For tells us how long it continues or lasts:

• I was ill for three days during my holiday and couldn't go out at all.
• I'll pop in and see you for a few minutes at some point during the afternoon.
• I've been working for this company for twenty five years.

Take care not to confuse for with since. Since is also used to measure the duration of an activity, but it describes the starting point up to a given time and is most often associated with present perfect and past perfect tenses:

• I've been working for the BBC for a long time - since 1978.
• As you get older, it becomes more and more difficult to make friends.
• We haven't seen much of him since his marriage to Julie last summer.

Note from the above examples that for is used with a wider variety of tenses than since.

until

We use until or till to indicate that something continues up to a particular point in time and then stops:

• Don't bother saving me any supper - I shan't be home till late.
• We had to stay in the exam room until the end of the exam. We couldn't leave early even if we had finished.
• I had no umbrella so waited until the downpour was over before I left the shop.
• We don't need to be at the stadium until the first race is over so we don't need to leave home till eleven o' clock.

by

We use by to indicate that something will be achieved before a particular time or at that particular time at the latest. Note the contrast between by and until in the final examples below:

• We have to be at the stadium by midday, so we should leave home by eleven fifteen.
• She had learnt to play the piano by the age of nine. By that age she could play almost any tune you asked her to.
  
• She learnt to play the piano until she was nine years old. Then suddenly and without warning, she quit.