Life for me ain't been no crystal stair.
It's had tacks in it,
And splinters,
And boards torn up,
And places with no carpet on the floor—
Bare.
But all the time
I'se been a-climbin' on,
And reachin' landin's,
And turnin' corners,
And sometimes goin' in the dark
Where there ain't been no light.
So, boy, don't you turn back.
Don't you set down on the steps.
Cause you finds it's kinder hard.
Don't you fall now—
For I'se still goin', honey,
I'se still climbin',
And life for me ain't been no crystal stair.
The mother shifts from describing life through indicative verbs in her  extended metaphor  to giving her son advice with imperatives. 
 The mother reveals her purpose with these lines: to convince her son to persevere in life despite its difficulty. The beginning of the poem is discouraging, and afterwards the mother talks about how she has dealt with her difficult life. But these lines reveal the tenderness and care the mother feels towards her son underlying this poem. 
 The mother uses  anaphora  to organize her advice, always beginning with “don’t you.”
Mother to Son