Kunal Saini

I can also speak English, and I score terribly in Grammar. But I know when my Grammar is wrong or if I constructed a sentence wrong.
If you mean it that way, then you don't have to worry about it. As Grammar in class 10 and above exams is just along the lines of finding out mistakes.
But if you mean it like you construct a sentence with wrong Grammar and people have to tell you when you are wrong, then you just simply have to listen to someone speaking English who can construct sentences with correct Grammar.
Do it in 2 ways:
1. Look at the top student's answers written in English. By top student, I mean top student in every subject. I have seen people scoring high in English with terrible Grammar who can't even write in English correctly outside of school work.
2. Watch English movies. And listen to dialogues carefully and try to predict what will happen next. Simply watching English movies is as good as not watching them. But when you try to think what will happen next, you are utilizing what you heard up until then, and you will learn and remember English much faster.
3. Learn about your spelling mistakes. Even after you learn to speak by first 2 steps, the biggest hurdle ahead is spellings. So, keep that in mind. And also try to look at the spelling mistakes in your friend's notebooks and answer sheets and discuss mistakes you both made in your papers.
4. Do these properly, and you might improve pretty decently before your first term exams. And your English teacher won't have any complaints with your answer sheets.
And whenever you speak or write English compare yourself with how the topper or someone who can converse properly in English wrote it, before long, your English will be top-notch even if you can't score well in a straightforward grammar test. But if your question is about how to score in Grammar, then, it's as tricky as scoring high in maths. Plus it's pointless because grammar-related problems are non-existent in 10th grade and above.
If you can learn to speak and write correct English without having to go too deep into Grammar then, you should do it. Cause, when you can speak and write correctly, no one is going to ask you about where the noun adjectives etc. are. As simple as that.