Question

it (rain) heavily last night

Answer 1

it had rained heavily last night.

Answer 2

Answer:  

'It (rained) heavily last night'

(Or)

'It (was raining) heavily last night'

Explanation:  

“It rained heavily last night” describes a completed event which uses the 'past perfect tense.'

• 'Past perfect tense' talks about the completion of an event in the 'past' before something other than it happened.    

The second sentence describes the event that was happened during the period of time. This sentence uses the 'past imperfect tense'.

• The 'past imperfect tense' is a combination of past tense and imperfective event (Continuing or repeating event).