I assume this is how you did it:
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At first, you could only speak in words.
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People began to talk to you in sentences.
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You started to speak (in simple sentences).
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People kept talking to you in sentences.
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Finally, you got really good at making sentences
As you can see, you learned to speak your first language by doing just two things:
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Listen to people speak correct sentences over and over.
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Speak the language over and over.