The lines that add to an ambiguous message in "The Road Not Taken" are:
- And sorry I could not travel both
- And that has made all the difference
- I doubted if I should ever come back
These lines convey uncertainty and reflect on the choices made, contributing to the ambiguity of the poem's overall message about decision-making and its impact on life. The line "Oh, I kept the first for another day!" does suggest a potential for revisiting choices, but it doesn’t necessarily add to the ambiguity in the same way as the others.