How to Clean Install macOS Sierra Imac 2011

Описание: You can use macOS Recovery, your computer's built-in recovery system, to reinstall the Mac operating system.

Press the power button on your Mac to turn it on, then release the power button and immediately press and hold both the Command (⌘) key and R key until you see an Apple logo or spinning globe.

Command + Option + R will reinstall the latest version of macOS compatible with your Mac (in most cases). Else, it will be the same as what’s mentioned below.
Command + Option + Shift + R will try to reinstall the version of macOS or OS X that was initially installed on your Mac when it was newly purchased. If the original version is unavailable, it will install the next closest version still available.

(The macOS High Sierra Recovery Tool is not working ) use Internet Recovery Mode on Intel-based Mac
And install Mac OS X 10.8