The subject of a sentence needs to agree with its  verb. Students often have problems when the subject is far from the verb.  In most cases, reading the sentence aloud will illustrate the subject,  which is heavily stressed,  though not the verb, which is weakly stressed.  Look at the following (altered) passage to see if you can spot and correct the 15 subject-verb agreement errors:

  HEREIN lies buried many things which if read with patience may shows the strange
     meaning of being black here in the dawning of the Twentieth Century. This meaning is
     not without interest to you, Gentle Reader; for the problem of the Twentieth Century
     are the problem of the color-line.
                                                                         1
       I prays you, then, receive my little book in all charity, studying my words with me,
     forgiving mistake and foible for sake of the faith and passion that is in me, and
     seeking the grain of truth hidden there.
                                                                         2
       I has sought here to sketch, in vague, uncertain outline, the spiritual world in which
     ten thousand thousand Americans lives and strive. First, in two chapters I has tried
     to show what Emancipation meant to them, and what was its aftermath. In a third
     chapter I has pointed out the slow rise of personal leadership, and criticised
     candidly the leader who bears the chief burden of his race to-day. Then, in two other
     chapters I has sketched in swift outline the two worlds within and without the Veil,
     and thus have come to the central problem of training men for life. Venturing now
     into deeper detail, I has in two chapters studied the struggles of the massed millions
     of the black peasantry, and in another has sought to make clear the present
     relations of the sons of master and man.
                                                                         3
       Leaving, then, the world of the white man, I have stepped within the Veil, raising it
     that you may view faintly its deeper recesses,—the meaning of its religion, the
     passion of its human sorrow, and the struggle of its greater souls. All this I have
     ended with a tale twice told but seldom written.
                                                                         4
       Some of these thoughts of mine have seen the light before in other guise. For kindly
     consenting to their republication here, in altered and extended form, I must thanks the
     publishers of The Atlantic Monthly, The World’s Work, The Dial, The New
     World, and the Annals of the American Academy of Political and Social
     Science.
                                                                         5
       Before each chapter, as now printed, stand a bar of the Sorrow Songs,—some
     echo of haunting melody from the only American music which welled up from black
     souls in the dark past. And, finally, need I add that I who speaks here am bone of the
     bone and flesh of the flesh of them that lives within the Veil?