Learn German: Ungeachtet der Ähnlichkeit fechten wir das Patent rechtlich an. - Regardless of the similarity, we are challenging the patent legally.

The preposition ungeachtet means regardless of or despite, and it requires the genitive case. Because Ähnlichkeit is a feminine noun, its definite article changes from die to der in the genitive case.

Why is the verb fechten placed before the subject wir?

The entire prepositional phrase Ungeachtet der Ähnlichkeit occupies the first position in the sentence. Since German main clauses require the conjugated verb in the second position, fechten must come immediately after the phrase, pushing the subject wir into the third position.

What is the base verb for fechten ... an and how is it used here?

The base verb is anfechten, which is a separable verb meaning to contest or challenge something, usually in a legal or formal context. Because it is a conjugated verb in a main clause, the separable prefix an is pushed to the very end of the sentence.

Could we use trotz instead of ungeachtet in this sentence?

Yes, you could say Trotz der Ähnlichkeit (Despite the similarity). However, ungeachtet is slightly more formal and elevated. It fits perfectly here because challenging a patent is a formal, legal action where high-register language is natural.