Young children have the ability to correctly hear every sound in every human language.1 As you grow, your brain forfeits the ability to hear sounds it doesn’t need so that it can process native language (NL) more efficiently. This means that there are sounds in your target language (TL) that you cannot currently perceive. Your brain will filter them out before you have a chance to notice them. All is not lost, however. The brain is perfectly able to regain the ability to hear sounds that it previously decided were unimportant.
By studying the phonology of your TL, you can accelerate your ability to hear these sounds. The goal is to reach the point where you can begin learning vocabulary, so you only need to learn the basics of your TL’s sound system. You won’t need to worry about high-level phonetic rules until Stage 4.
How to Study Sounds
Search the internet for explanations of the language’s phonology (sound system). Research which vowel and consonant sounds exist, and roughly how they are formed with the mouth. Use a few different resources. Look for resources that both let you hear the sounds and illustrate how they are articulated (mouth shape, tongue placement, etc.).
If your TL has any other core phonetic features, such as stress accent, pitch accent, or tones, learn about those as well.
During active immersion, be on the lookout for these sounds. Use the exercises described in 1A Active Immersion.
Footnotes:
1: “Why the baby brain can learn two languages at the same time”, theconversation.com