-If your green card application is as the dependent on your spouse, then say "No" in the 3a, "No" in the 3b, and "Yes" in the 3c.
If yes, are there any problems for me to get H4 extension
-No problem at all.
and what should I explain on seperate sheet of paper?
-Just write one sentence on the space beside this question such as "I-485 was filed on mm/dd/yy; receipt# ......"