Hello mate,here is your answer!
Inspite of trying so hard,they couldn't open the door.
Hope it helps you.
However hard they tried, they couldn't open the door.
This sentence if started with in spite of will be as follows:
In spite of trying so hard, they couldn't open the door
The meaning of both the sentences is same.
However in the first sentence has been replaced with in spite of in the second sentence.
vycych jcjv vhyg by hyv jgj