Learn Russian: Не бойся, это просто огонь.

This matches the rule for negative commands! When you tell someone not to do something, Russian almost always requires the imperfective aspect. Plus, the verb бояться (to be afraid) describes a continuous state of feeling rather than a single completed action.

Why does the verb end in -ся instead of a regular command ending?

The base verb is бояться, which is a reflexive verb (identified by the -ся or -сь at the end). When you form the informal ты (you) command for this verb, you use the imperative ending -й and attach the reflexive particle -ся, giving you бойся. If you were speaking formally or to multiple people, it would be бойтесь.

Огонь is a masculine noun. Why do we use the neuter word это instead of этот?

Because это is acting as a fixed subject meaning 'this is' or 'it is'. When used this way to introduce or identify what something is, the word это never changes its form, regardless of the gender of the noun that comes after it.

I learned that просто means 'simply'. Is it translated as 'just' here?

Yes! In Russian, просто is very versatile. While it does mean 'simply', in everyday conversational contexts like this one, it is the exact equivalent of the English word 'just' (meaning 'it is nothing more than fire').