Ear training is any exercise specifically designed to improve your ability to hear and distinguish the sounds of the target language.
Every language has its own set of sounds, and your native language likely doesn't use all of them. Your brain has learned to filter out unfamiliar sounds, so you need to actively retrain your ears to hear them.
Minimal pairs training: Listening to pairs of words that differ by only one sound (like "ship" vs. "sheep" in English) and practicing telling them apart. Many languages have sounds that seem identical to non-native ears but are distinct to native speakers. Minimal pairs training builds the ability to hear those distinctions.
Sound study: Learning about the phonology (sound system) of your target language — what sounds exist, how they're produced, and how they differ from your native language's sounds. Even a brief overview makes a noticeable difference.
Targeted listening: Focusing specifically on hard-to-hear sounds during your regular immersion. Once you know what to listen for, you start hearing it everywhere.
Ear training has a dual benefit: it improves your listening comprehension (you can understand more of what people say) and it improves your pronunciation (you can't produce sounds you can't hear).
Even small amounts of ear training — 10-15 minutes a few times a week — produce noticeable results. You don't need to dedicate huge amounts of time to it.
Good resources for ear training depend on your specific language. Common recommendations include Fluent Forever's pronunciation trainers, The Mimic Method, language-specific minimal pairs exercises, and the phonology sections of Wikipedia articles about your target language.
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Light ear training begins in Phase 1 (as part of Sound Study) and becomes more important in Phase 3 when you're focused on building listening ability.