if only引導的是什麼從句

例句：

If only you had told me that some time ago.

要是你之前告訴我那件事該多好。

If only he'd remembered to send that letter.

要是他沒忘記發那封信就好了。

If only I could have helped put matters right.

要是我當時能幫着把錯誤糾正過來就好了。

'If only I had known you then,' he said wistfully.

“要是我那時認識你就好了。”他嚮往地説道。