Hi !!
1- The town wouldn't have been destroyed if the volcano <u>hadn't been</u> so destructive.
2- If the inhabitants had evacuated early enough, the terrible disaster <u>wouldn't have been averted</u>.
3- If the weather <u>is</u> gloomy, we will stay in the hotel and read.
4- If <em> </em><u>heard </u>a loud, ......, I would return to the hotel immediately.
5- A tsunami <u>would be set off</u> if a nearly volcano erupted.
Explanation
if + present ---------> main clause = future
if + simple past ---> main clause = would
if + past perfect ----> main clause = would have
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