Please sir explain this question in brief as I have a little doubt.(Why we can't solve it like 1000-q_10uF=0 as Point E is earthed and why potential of 30uF is in negative as in other circuit diagram it is not negative.)

Because the potential at positive terminal is not absolute 1000V and that at negative terminal is not 0V absolutely.

Only the difference between the two points would be 1000V. SInce E is earthed, the potential at positive terminal would be postiive(<1000V) and that at negative terminal negative.