1. it happened earlier than other events in the past
英語 "という言葉Past Perfect"(it happened earlier than other events in the past)集合で発生します。
Love Research2. a letter had been written
英語 "という言葉Past Perfect"(a letter had been written)集合で発生します。
czasy i gramatyka3. Past Simple
4. Present Perfect
5. Past Perfect
Use regret + gerund, if only + past perfect or wish + past perfect to say we want something in the past to have been different.