"Now, of my threescore years and ten,
Twenty will not come again."
A score is equal to twenty years. Three times twenty is sixty. Add another ten years and what the speaker is predicting here is that he will live to about seventy. By today's standards, that seems a pessimistic figure, but at the time the poem was written, living seventy years was actually optimistic. In addition to telling the reader how long he expects to live, this quote gives us the age of the speaker and situates the context. He is a twenty-year-old male contemplating the passage of a lifetime.
"Loveliest of trees, the cherry now"
The opening line of the poem immediately answers the question of what is the loveliest of trees. The cherry tree features pointy oval-shaped leaves. and flowers typically start to bloom in April. In the second stanza, the speaker references that he expects to see fifty more springs. In the first stanza, he mentions that the tree has bloomed. Thus, one can infer from these clues that the tree is at its loveliest for the speaker sometime in the spring. Spring is also a metaphorical allusion to the time in which the poem is set.
"About the woodlands I will go
To see the cherry hung with snow."
While the cherry tree in full bloom in the spring may be the favorite time for the speaker to enjoy the trees, he will make sure it is not the only season. In the poem's closing lines he makes a promise to himself to take advantage of what he sees as the limited time he has left in his prediction of living another fifty years. That advantage will take the form of enjoying the trees even at the height of winter when the flowers are gone and the limbs are adorned with snow. This reference to winter is also a metaphorical allusion to the speaker's inevitable aging.
