STUDENT: When I forgot my father's birthday,
CORRECT: When I forget about my father's birthday,
EXPLANATION:

a. If we use WHEN instead of IF, we are talking about a possible action that could happen in the future. Thus, we need to use the present tense of the verb on the first clause.

b. Do not forget to use ABOUT which means "it is related to the event that you are talking about."

STUDENT: I will say sorry and I will have a party today.
CORRECT: I will say sorry and I will have a party on the same day.
EXPLANATION:

a. If you say TODAY, that means you will have the party this exact day even if it's not your father's birthday. However, when you say ON THE SAME DAY, It means you are going to have the party once you remember your father's birthday.

STUDENT: I also write card ( Father, I'm Sorry I forgot your birthday but I love you forever.)
CORRECT: I will also write a card that says: "Father, I'm sorry I forgot about your birthday. But I love you forever."
EXPLANATION:

a. If we are talking about a possible occurrence in the future, we use WILL.

Hello, Elsa! I think it is impossible for you to forget about your father's birthday, though. Right?

- Teacher Macy