When determining rhyme scheme, you need to examine the final word in each line of the poem. The last word in the first line is indicated by the letter a. If the final word in the second line rhymes with this word, it also gets a letter a. If it doesn't rhyme, it is indicated with a letter b. You proceed through the entire poem, matching rhyming words with matching letters. Thus, the rhyme pattern of this poem looks like this:

Whose woods these are I think I know. a
His house is in the village though; a
He will not see me stopping here b
To watch his woods fill up with snow. a

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

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

The woods are lovely, dark and deep, d
But I have promises to keep, d
And miles to go before I sleep, d
And miles to go before I sleep. d

It's also important to note that not all poems have a rhyme pattern; however, it's clear to see the visual pattern in "Stopping by Woods on a Snowy Evening." The third line in each stanza becomes the rhyming sound for the following stanza.

Poets create a particular rhyme pattern to achieve various effects, so it's worth considering how this rhyme scheme affects the poem as a whole. In this poem, the rhyme scheme mimics the halting and continuing patterns of the speaker. The third line, not matching the others in each stanza, gives a moment of pause before giving the poem momentum as it moves into the following stanza. This reflects the way the speaker halts in his journey before remembering the need to continue through these snow-filled woods. In the final stanza, no new rhyme pattern is introduced, which brings the poem to a sense of completeness. The repetition of the same sound in all four lines also emphasizes the hollowness in that long e sound, reflecting the speaker's own hollow darkness as he trudges forward with much to do before he can "sleep."

Robert Frost's "Stopping by Woods on a Snowy Evening" appears, at first glance, to be deceptively simple in its meter and rhyme scheme. Upon closer inspection, however, we can see that something more complex and clever is happening, with rhyme being used cleverly as a thread to connect the four stanzas of the poem.

All the stanzas of the poem conform to the same meter. Of the stanzas, the first three have similar rhyme schemes: the first, second, and fourth lines all rhyme, while the third does not. This sound from the third line then follows through to the next verse, where it becomes the rhyme aaba, bbcb, ccdc.

This serves to highlight the word from the previous stanza which has now provided the focal sound for the stanza to follow. For example, "here" in the first stanza is isolated and has no rhyming pair; in the second stanza, it is echoed in the line endings "queer," "near," and "year."

The effect of this continuity of sound from one stanza to the next is, of course, most keenly felt in the final stanza of the poem. In the fourth stanza, all the lines rhyme (dddd). The sound suggested in the third stanza by the word "sweep" is continued through into the fourth stanza, where it rhymes with "deep," "keep," and then, twice, "sleep."

The change in rhyme scheme here has a cumulative effect, which could be said to represent the increasingly "deep" snow as it falls. The repetition of the word "sleep," too, in combination with the gentle sound of "sweep," is sonorous and somnolent, drawing the poem to a quiet and sleepy conclusion.

Whose woods these are I think I know.      A
His house is in the village, though;            A
He will not see me stopping here               B
To watch his woods fill up with snow.         A

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

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

The woods are lovely, dark and deep,         D
But I have promises to keep,                     D
And miles to go before I sleep,                   D
And miles to go before I sleep.                   D

The rhyme scheme in the first stanza is AABA. In the next stanza the poet picks up the B ("here") and the rhyme scheme for that stanza is BBCB. Then he picks up the C ("lake") and the rhyme scheme for the third stanza is CCDC. And the final stanza picks up the D ("sweep"). The final stanza is straight DDDD. The effect of this rhyme scheme seems to be to mimic the impression of gently falling snowflakes, and the final stanza seems to be intended to suggest the buildup of snow on the ground.

Of course, there is a great deal more to the poem than the impression of falling snowflakes, but the speaker does say that his purpose in stopping by these woods on a showing evening is to "watch his woods fill up with snow." So the reader may have the illusion of watching the falling snowflakes along with the speaker which is enhanced by the rhyme scheme. There is a definite falling effect in the AABA, BBCB, CCDC, and then an impression of deepening snow at the "bottom" of the poem. The snow is not falling heavily. The speaker describes the weather conditions as "easy wind and downy flake." The flakes are floating down slowly, and the rhyme scheme seems to suggest that familiar sight. The B in the first stanza might be considered a drifting snowflake which lingers throughout the second stanza, and so on. If the snow were falling heavily, the speaker probably would not have stopped there that night. 

The repetition of the words "And miles to go before I sleep" with the two D rhymes at the very end has suggested to some readers that the speaker is having sombre thoughts and might even be thinking of walking out into the beckoning woods and letting himself freeze to death. Frost repeatedly denied this. The thought that he has miles to go before he sleeps could also be interpreted to mean that he expects to have a long life ahead of him with many things he wishes to accomplish--which in fact was the case. He wrote the poem in 1922 and died in 1963. He was nearly eighty-nine years old.