If only I had (past participle)

If only I had (past participle) is used to express regrets. It means "I wish I had (past participle)".

If only I had (past participle).

past participle	sentence
written it down	If only I had written it down.
been more careful	If only I had been more careful.
not spent so much money	If only I had not spent so much money.
not said that	If only I had not said that.