It after when?

[Hobby]

English Grammar Tests, Elementary Level

ESL/EFL Test #44 "Basic Weather Expressions", question 5

I like it ......... it rains.

(a) when
(b) who
(c) why
(d) whom

English Grammar Tests, Elementary Level

ESL/EFL Test #44 "Basic Weather Expressions", answer 5

I like it when it rains.

Correct answer: (a) when
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why use it before when?

Hobby

[Mister Micawber]

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'It' is after 'when' and it used as the subject of an impersonal verb: 'It is snowing'.
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Native English teacher at Mister Micawber's