Which linguistic technique is primarily used in Duolingo's famous slogan 'Learn a language for free. Forever.'?

In the competitive world of language learning apps and tools, brand slogans serve as powerful linguistic hooks that capture both a product's value proposition and its personality. These carefully crafted phrases employ specific linguistic techniques to create memorable, emotionally resonant connections with customers. Test your knowledge about how language technology brands leverage linguistic principles in their marketing slogans!

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Parallelism and temporal contrast (free vs. forever)
Rhyming couplet with alliteration
Imperative mood with interrogative conclusion
Extended metaphor with rule of three