Improving sentence totally depends on the sentence structure, The making a sentence structure correct make the sentence more understandable and appreciative. But above that you have to understand one thing If you can make a sentence in 5 words do not try to make it 10 words. Like very short example : 

What is your name?
Ans : My name is Arundhuti. (if you say a sentence like this then its totally not impressive)

What is your name?
Ans: Arundhuti.

So try to make sentence short as possible its also try to put phrasal verb. Thats much more apprititable then using normal words. But between all of them remember to put correct tense and grammar. Read newspapers, Novels and Watch movies that helps alot. 
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When we write we get time to think and we write properly. But when we speak, we have to answer immediately, and mistakes occur. We have lots of practice of writing . So it makes our writing accurate. That's why u should speak more and more and graudually your mistakes would reduce.