Sir, Please check my answer which I have Joined the two sentences in each case with the help of coordinating/ correlative conjunctions.
Q.a) i) I tried to walk as briskly as I could.
ii) I could not reach the concert in time.
My Answer: I tried to walk as briskly as I could but I could not reach the concert in time.
Q b) i) You come to school early and finish the work.
ii) You stay back after school and finish the work.
My Answer: You come to school early and finish the work and you stay back after school and finish the work.
Q c) i) Cut down on your chocolate intake.
ii) You will ruin your teeth.
My Answer: Cut down on your chocolate intake otherwise you will ruin your teeth.
Qd) i) I had told him not to leave his little brother alone.
ii) He left him at home and went out to play.
My Answer: I had told him not to leave his little brother alone but he left him at home and went out to play.
Q e) i) He picked up the letters lying near the gate.
ii) He went up the stairs to unlock the door.
My Answer:He picked up the letters lying near the gate and he went up the stairs to unlock the door.
Q f) i) His parent did not know where he had gone.
ii) His teachers did not know where he had gone.
My Answer: His parent and his teachers did not know where he had gone
Q g) i)He spent all of his own money .
ii) He also spent the money he had borrowed from his brother.
My Answer:He and his brother spent all of his own money that he had borrowed from him.
Most of the answers provided by you are incorrect. However, I am providing the solution for all the sentences.
a) I tried to walk as briskly as I could but I could not reach the concert in time.
b) Either you come early and finish your work or you stay back after school and finish the work.
c) Cut down your chocolate intake or you will ruin your teeth.
d) I had told him not to leave his little brother at him but, he left him and went out to play.
e) He picked up the letters lying near the gate and went up the stairs to unlock the door.
f) Neither his parents nor his teachers know where he had gone.
g) He had spent not only all of his own money but also the money borrowed from his brother.