ORA-00907 MISSING RIGHT PARENTHESIS

OK this is oracle sql error. the first step to trouble shoot it is by spotting what the error said: looks for missing right parenthesis.
But unfortunately, this is not the only case for this error thrown out. Even when we match the count of left and right parenthesis, error ORA-00907 still can be thrown out. some of the cases are:
1. In program, for multi lines sql query, if you put comment it will throw error. ex.
sql = ” select a,round(b,2 –comment ”
sql &= ” ) from table ”