I was thinking of love songs to use for my Valentine's Day lesson, and this old classic came to mind. Another site (esl-lounge) has a huge list of songs and what grammatical & linguistic points can be taught within each. It reminded me that this song uses a lot of past modals, which is what my B2 class had just finished. For them, filling in the gaps in this song was relatively easy. Perhaps lower levels could do it, too, and listen a few times. Maybe it can be a segue into a conversation about things we regret in life and love.