English Is there anything wrong with the sentence It'll start raining in the afternoon? Do you use the form start doing?

1. It is best to use start to rain.

Item 4 of the knowledge points you listed says: When something is the subject, it is best to use start/begin to do sth instead of start/begin doing sth.

it here refers to the weather, which of course belongs to things.

2. However, if it is an informal situation such as speaking, there is nothing wrong with using raining. The meaning of the sentence is smooth and can be understood, and there is no need to be obsessed with grammar.

Nowadays, many teaching materials are more inclined to speak, so the poster really does not need to pursue these minutiae.