rain / raining

1a. It’s starting to rain.
1b. It’s starting raining.
2a. It started to rain.
2b. It started raining.
3a. It’s beginning to rain.
3b. It’s beginning raining.
4a. It began to rain.
4b. It began raining.
Please correct the above.

1b and 3b are incorrect. The two -ing forms don’t work together.