Put the verbs in brackets into the correct tense to form conditionals. Write down only the correct answer. Use the full form of the verbs. I would gladly live on the moon if they ever _______ (colonise) it.

The sentence is a mixed conditional type 3, where we consider a hypothetical past event affecting a hypothetical present situation. The structure is: If + past perfect, would + base form/could + have + past participle.

In the given sentence, we have 'I would gladly live on the moon', which indicates a hypothetical present situation. Therefore, we should use the past perfect tense in the 'if' clause.

So the correct answer is: had colonised.

I would gladly live on the moon if they ever had colonised it.

**Translation into Russian:**
Я бы с удовольствием жил на Луне, если бы они когда-нибудь её колонизировали.

**Answer:** had colonised