The sentences below contain one mistake each. Correct it:
a. I have once studied the guitar for three years.
b. I am here since three o’clock, but nobody has come yet.
c. When she arrived, I was waiting for three hours and a half.
d. I have seen him three days ago.
e. Will you have been having dinner out tonight?
f. I knew she will arrive before long.
g. I think it’s raining tonight.
h. You are hating this party.
i. I am seeing a rabbit over there.
j. He always forgets people’s names.