(i) 'I' is the author Robert Lee Frost. It is the winter season and it is the longest and the coldest evening of the year. The line 'To watch his woods fill up with snow' and 'Frozen lake' is the evidence given in the poem.
(ii) The horse of the narrator has made the author aware of his mistake in stopping there. The horse does so by shaking his harness bells. Horse wants to ask if there is some mistake that they have stopped between the woods and the frozen lake without a farmhouse near.
(iii) The three sounds heard by the author are the shaking of the harness bells by the horse, sweeping of the downy flakes and easy wind.
(iv) The poet has said that he knows the owner of the woods. He says in a voice of despair that the owner lives in the village away from the woods and he is not able to admire the beauty of his woods being filled with snow.
(v) The poet finds the woods to be beautiful and charming. The woods appear to be restful, seductive and lovely to the poet. When taken at a symbolic level, the woods are opposed to the promises which the poet must keep. Woods represent sensuous enjoyment (lovely), the darkness of ignorance (dark) as well as the dark inner self of man (deep).
The last two lines means that the poet Robert Frost has to fulfill his promises and has to travel a lot of distance before he can rest. Sleep could also be interpreted as a metaphor for death.
Poet attaches the moral tag that in life one must do one's duty and carry out one's obligations.
