Learn Hardcore Finnish: Älä pelkää heitä.

Älä is the negative imperative for the 2nd person singular: it means don’t (you). In commands and prohibitions, Finnish uses special negative imperative forms:

2nd person singular: älä (Don’t you…)
2nd person plural: älkää (Don’t you all…) By contrast, ei is the ordinary negative verb used in statements and questions (e.g., En pelkää = I don’t fear), not in direct commands.

Why is the verb pelkää shaped like that (long ää, no -t)?

The dictionary form is pelätä (to fear). In the 2nd person singular present, you’d say pelkäät (“you fear”).
In the imperative (a command), the 2nd person singular form is pelkää! (“fear!”).
In a negative imperative, you use älä
- the verb’s imperative/connegative form: Älä pelkää (“don’t fear”).
The ää is a long vowel; length matters in Finnish. pelkää is pronounced with a long final vowel [æː], not short.

Why is it heitä and not he?

Because pelätä governs the partitive case for its object. The pronoun he (“they”) in the partitive plural is heitä, so you get pelkää heitä (“fear them”).
This also aligns with a general Finnish tendency for negative clauses to take a partitive object.

Does heitä ever mean “throw”? How do I know it means “them” here?