Learn French: Qu'il me poignarde dans le dos avec une telle audace ! - That he would stab me in the back with such audacity!

This is an example of the emphatic or indignant independent subjunctive. You can use Que followed by the subjunctive all on its own to express strong disbelief, outrage, or indignation. It implies a hidden main clause like I cannot believe that... and translates well to the English exclamation That he would...!

Since poignarde looks exactly like the normal present tense, how do we know it is in the subjunctive?

For regular -er verbs, the present subjunctive and present indicative share the exact same endings for most subjects, including il. We know this is the subjunctive because of the grammatical structure: an exclamation beginning with Que plus a subject almost always triggers the independent subjunctive.

Why does the French say dans le dos (in the back) instead of dans mon dos (in my back)?

French rarely uses possessive adjectives (mon, ton, etc.) with body parts when the person affected is already clear. Here, the object pronoun me earlier in the sentence establishes that it is your back being stabbed (il me poignarde = he stabs me). Therefore, you just use the definite article le for the body part itself.

Why is there an une before telle audace? Doesn't telle already mean such?