I've edited the answers. Could someone please see if there's still something lacking, or if there are any mistakes left?

Here's the poem, the questions; answer follow.

Stopping By Woods On A Snowy Evening Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.

My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.

He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.

The woods are lovely, dark and deep.
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep. Stanza # 04(All the questions below are related to Stanza No. 4)

Why can’t the poet wait to enjoy the beauty of the woods?

He can’t wait to enjoy the beauty of the woods, because he has too many responsibilities to fulfill, and has a long distance to travel before he can rest for the night.

Why does he repeat the third line?

He repeats the third line in order to imprint it on the reader’s memory, as well as to foreground the presence of a non-literal meaning, but not the meaning itself. In other words, the repetition gives the reader of the poem the opportunity to go beyond the literal into the figurative. Furthermore, the poem has a strict rhyme scheme. The first three stanzas go: aaba bbcb ccdc. Three lines rhyme; the line that doesn't rhyme provides the rhyme for the next stanza. There are just enough stanzas to establish a pattern. The fourth stanza is the last. Since there is no stanza after the last one, the third line cannot have a none-rhyming line. Now, instead of having just three lines, or just four rhyming lines, the poet chose to repeat the last line. This gives a powerful feeling of "closure"; not only is there no new rhyme, there isn't even a new line. The first "sleep" disrupts the expectation (we would expect a non-rhyming line); the repetition wraps up the poem.

Which letters have been repeated in the last stanza?

Since the rhyme is [...i: p] in the last stanza, the repeated letters that are most obvious would be: "e" (as "ee", sounding [i:]; it also appears in "lovely" or "promises", but the letter sounds differently there) and "p". It's true that the letter "o" is also repeated a lot, but it sounds differently in "woods", "to", "lovely" and "promises".

Why have they been repeated?

They might have been repeated to produce a profound impression.

(Edited to remove HTML-- it denies replies to this post.  MM)