The document discusses different structures used to express regrets about past actions or situations in English. It explains that "wish + past perfect" and "should have + past participle" are used to express simple regrets. "Ought to have + past participle" is also used but cannot be used in negative statements or questions. "If only + past perfect" expresses strong regret and is used in past unreal conditional statements with a result clause. Examples are provided to illustrate the usage of each structure. The document concludes with practice problems asking the reader to form regret statements based on given scenarios.