to rain
Pleuvoir is a French verb that means 'to rain'. It is an impersonal verb, which means it is used without a subject. For example, 'Il pleut' means 'It is raining'. Pleuvoir can also be used in the past tense, as in 'Il a plu hier' which means 'It rained yesterday'.
It is going to rain tomorrow.
This sentence uses the future tense to indicate that it will rain in the future, specifically, tomorrow.
It has started to rain.
This sentence uses the past tense to indicate that the event, the rain, started in the past.
When it starts to rain, I go home.
This sentence uses the present tense and actually relates two events together. When the first event (the start of rain) happens, the second event (I go home) happens.