Learn Russian: Не жди неудачи и не бойся темноты! - Do not expect bad luck and do not fear the dark!

This is the main rule of this lesson! Verbs of expectation like ждать (to expect) and verbs of fear like бояться (to fear) take the Genitive case for their objects. For ждать, we use Genitive especially when expecting something abstract (like неудача, meaning bad luck) or when the verb is negated. For бояться, the object you fear is always in the Genitive, so темнота (darkness) becomes темноты.

Why do we use the imperfective imperative forms жди and бойся?

Negative commands (telling someone not to do something) almost always use imperfective verbs in Russian. Since you are telling someone to not engage in the general behavior of expecting or fearing, imperfective is the natural choice here.

If ждать usually means to wait, why is it translated as to expect here?

The verb ждать covers both waiting for and expecting. When the object is a physical person or transport, it usually means wait. When the object is an abstract concept or event, like неудачи (bad luck), it translates much better as expect.

Why does the reflexive ending on бойся use -ся instead of -сь?