Both would be fine.
Student or Learner
1. When did it start raining? > It started raining an hour ago
2. We had been playing tennis for about half an hour when it started to rain hard
Why don't we write started raining?(#2)
We must write "started to"?(#2)
I would use "started raining" in #2. The original is perfectly correct so it's just personal choice, but I would say "We'd been playing tennis for half an hour when it started raining".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.