Line 20 is important to the poem because it helps 3) describe how the speaker has changed his or her outlook
What happens in the line ?
In the previous lines, the speaker has been talking about how their love for their partner has made them feel complete and whole. They have said that they used to be "weaned" on "country pleasures", but now they see that all of those pleasures were just "fancies" compared to the love they feel for their partner.
In line 20, the speaker says that "If our two loves be one, or, thou and I / Love so alike, that none do slacken, none can die." This suggests that the speaker now believes that their love for their partner is so strong that it can never die. They believe that they are two halves of a whole, and that they will be together forever.
Note : The poem is The Good-Morrow by John Donne