Learn Russian: Я ни за чем не прячусь, здесь только песок и ветки. - I hide behind nothing, there is only sand and branches here.

When a negative pronoun like ничто (nothing) is used with a preposition, the preposition steps right into the middle, splitting the pronoun. Instead of putting the preposition first, Russian splits the word into ни + за + чем.

Why do we use чем inside ни за чем instead of что?

The preposition за requires the Instrumental case when talking about location (being located behind something). The Instrumental form of что is чем, which is why the split pronoun uses that specific form here.

Since we already said ни за чем (behind nothing), why do we add не before the verb?

Russian has strict rules on double negatives! Whenever you use a negative word that starts with ни- (like никто, ничто, or ни за чем), you must also make the main verb negative with не. Literally, this sentence says 'I behind nothing do not hide'.

Why does the verb прячусь end in -сь?

This is a reflexive verb. The base verb прятать means to hide an object (like hiding a book or a treasure). Adding the reflexive ending -ся (which shortens to -сь after the vowel in прячу) makes the action happen to the subject, meaning 'to hide oneself'.