Learn German: Den Meißel fest haltend, betrachtet der Bildhauer den Marmor. - Holding the chisel firmly, the sculptor looks at the marble.

This is a free participle clause. It uses the present participle haltend (holding) to express an action happening at the exact same time as the main verb (betrachtet). It works like an adverbial clause, equivalent to saying 'While he holds the chisel firmly...' (Während er den Meißel fest hält, ...).

Why does the main clause start with the verb betrachtet instead of the subject der Bildhauer?

In German, the conjugated verb must be in the second position of a declarative main clause. The entire participle clause (Den Meißel fest haltend) acts as a single block of information occupying Position 1. Therefore, the verb (betrachtet) must immediately follow it in Position 2, pushing the subject (der Bildhauer) to Position 3.

Why doesn't haltend have an adjective ending, like -er or -em?

Participles only take adjective endings when they are placed directly in front of a noun to describe it (for example, der haltende Mann). In this sentence, haltend is used as an adverbial phrase to describe how the sculptor looks at the marble. Because it acts like an adverb here, it remains in its base, uninflected form.

Could I use the past participle gehalten here instead to mean 'having held the chisel'?