how can I reduce the grammatical mistakes in my speaking and writing?

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The only way is to get more practice and get feedback on your ability. Try watching some English movies 2 times. Once with subtitles on so you can get a good idea of the meaning, and again without the subtitles so you can concentrate on listening to how the people are using the language. Just so you know, your question should be, "How can I reduce grammatical mistakes in my speaking and writing?" You don't need "the" here because "grammatical mistakes" is plural and usually plural words don't need articles (a, an, the).