Grammar problem

[Unregistered]

Hi! I always wanna learn more about grammar, but I also like the informal ones! Tee-hee. These are my questions: (Please check the grammar)
1. Can you help me (to) do this? (?) Is “to” necessary?
2. Everyone has (its/his/their) own secret.
(?) Which one is correct, “its”,”his” or “their”?
3. I heard that this game (is/was) fun.
(?) Because “heard” is in the past, so do I need to change the ‘verb’ behind into past?
4. Which one is more suitable, “the commonest” or “the most common”?
5. What is the meaning of “You gotta get it down”?
6. Susan, Anton, or Tony (is/are) allowed to come.
(?) Which one is correct, "is" or "are"?
Thanks..

[Teia]

Hi! I always wanna learn more about grammar, but I also like the informal ones! Tee-hee. These are my questions: (Please check the grammar)
1. Can you help me (to) do this? (?) Is “to” necessary?
2. Everyone has (its/his/their) own secret.
(?) Which one is correct, “its”,”his” or “their”?
3. I heard that this game (is/was) fun.
(?) Because “heard” is in the past, so do I need to change the ‘verb’ behind into past?
4. Which one is more suitable, “the commonest” or “the most common”?
5. What is the meaning of “You gotta get it down”?
6. Susan, Anton, or Tony (is/are) allowed to come.
(?) Which one is correct, "is" or "are"?
Thanks..

1. Can you help me do this? - the preposition to is not necessary.
2. Everyone has his / her own secret.
3. I heard that the game was fun. - the game played at that time.
I heard that this game is fun. - in general
4. The commonest. More common is used as well.
5. You have to get it down. You`ve got to get it down.
6. Susan, Anton, or Tony is allowed to come. - rule of proximity.