Both "fast" and "quickly" are correct and natural in this sentence, but "quickly" is more commonly used and preferred in this context. Here's why: 1. "Quickly": - It’s an adverb and directly describes how the action (get used to new things) happens. - It sounds slightly more formal and is more typical in written and spoken English for this type of context. 2. "Fast": - While "fast" can function as both an adjective and an adverb, it’s less common as an adverb in this specific expression. - It’s more casual and might feel slightly less polished. Tip: Use "quickly" when you want to sound more precise or formal. Use "fast" for casual, conversational tone, especially when emphasizing speed.

They’re both correct (I think that ‘fast’ is more US English) but ‘quickly’ is more natural sounding as an adverb here.