Learn French: Impuissant, le pauvre garçon s'acharne pour obtenir sa friandise. - Helpless, the poor boy struggles to get his treat.

When pauvre is placed before the noun, it means pitiable or unfortunate, which fits this context perfectly. If it were placed after the noun (le garçon pauvre), it would specifically mean financially poor.

What is the exact nuance of the verb s'acharner?

While translated simply as struggles here, s'acharner carries a much stronger nuance of relentless, almost desperate persistence. It implies putting a fierce, stubborn effort into doing something, often against the odds.

How could I use the current lesson's grammar to say the poor boy 'does nothing but struggle'?

You would use the structure ne faire que followed by the infinitive. The sentence would become: Impuissant, le pauvre garçon ne fait que s'acharner pour obtenir sa friandise.

Why is Impuissant just floating by itself at the beginning of the sentence?

This is a detached adjective describing the subject (le pauvre garçon). Placing it at the very beginning sets the tone and emphasizes his state of being while he performs the action. It translates smoothly to helpless or being helpless.