In English, it means weather, such as rain, but under what circumstances?

In English, it means weather, such as rain, but under what circumstances should you add y or s or ing after it?

Excellent solution

1)rain is usually used in the present continuous tense when it is used as a verb.

It is raining.

When rain is used as a noun,

There is a heavy rain.

3) Sometimes the adjective rain is also used, so it should be used as a predicative after the be verb.

It rained today.

The first and third expressions have different emphases. The continuous tense emphasizes that it is raining at present, and the usage of the third adjective means that it is raining today, but it is not necessarily raining at this moment.