A. learn
B. learned
C. have learned
D. learning

A. do
B. does
C. did
D. done

A. explains
B. explained
C. is explaining
D. explaining

A. sleeps
B. sleeping
C. is sleeping
D. was sleeping

A. is
B. was
C. are
D. be

A. have
B. has
C. had
D. having

A. has not
B. have not
C. did not
D. was not

A. have
B. had
C. has
D. was

A. will
B. must
C. may
D. could

A. should
B. may
C. must
D. can