The problem of French yin and yang? Personal pronouns?

Except for some rules, some yin-yang changes of 1 are irregular and can only be memorized. The two situations you mentioned are irregular and need your daily accumulation.

2 il y a and il fait are completely different.

What does Ilya mean?

There is a storm in Ilya.

Another example is Hier, il y avait du brouillard. It was foggy yesterday.

If the weather is how to use il fait.

The weather is very good.

For example, rain, Adu wind and cold.

It's raining, windy and cold.