Detailed explanation
During, over and by all relate an event to time, but they encode different relationships. During places an event inside another event or bounded period. It answers the question “when within this period did something happen?”: during the lecture, during the winter, during the final phase. It does not normally describe the full duration of the action; for that meaning, use for: The meeting lasted for two hours, not during two hours.
Over presents a development, repeated activity or accumulated change across a span of time. It is especially common with gradual processes: prices rose over the decade, several faults emerged over the course of testing, the team met repeatedly over three months. Over can sometimes overlap with during, but it often highlights extension or evolution rather than mere location within a period.
By marks a deadline or latest point. “Submit the form by Friday” means no later than Friday. It differs from until, which describes continuity up to a point: We worked until Friday. By the time introduces a later reference point from which an earlier action is viewed: By the time the auditors arrived, the files had been organised. In future contexts, the future perfect often combines naturally with by: We will have completed the review by June.
Formal writing also uses expanded phrases such as during the course of, over the following weeks, by the end of, and by which time. These expressions make chronology explicit, but they should not be added mechanically. Choose the phrase that expresses whether the action occurred within a period, developed across it, or had to be complete before a deadline.
To check your choice, reformulate the sentence. If “at some point within” preserves the meaning, during is likely. If “throughout or progressively across” works, use over. If “not later than” is accurate, use by. When none of these paraphrases fits, another preposition such as for, until, since or within may be required.
How to form it
Use during + noun phrase for an event or bounded period: during the inspection. Use over + noun phrase for change, repetition or activity extending across a period: over the next six weeks. Use by + time expression for a deadline: by 5 p.m.; use by the time + clause to establish a later reference point.