i would like to know if we write articles using the fixed keyword instead of using the preposition for it good for SEO keyword density?
While it may be a good idea to write for humans or for search engines, it's a great idea to consider writing for both. The first rule of great writing is this:
Don't write to be understood; write so that you cannot be misunderstood.
Always assume that a sentence will be taken out of context or be used by itself to represent the whole of paragraph. If you use too many "he/she/it/them/those/us/thing/stuff/do", you will not be understood.
Which is the better sentence? Which one appeals to both humans and search engines? Which one is most likely to be understood out of context?
1) He gave her the stuff to make more things.
2) John gave Sarah the AC/DC albums to create additional wall photos.
Write the article so it makes sense to human beings. If you start writing content so it reads like what you think the search engines are looking for you're going to end up with bad content. Bad content makes getting links much more difficult to do. It also makes people leave your website which is also against your goals.
Anyway, search engines are smart enough to know synonyms, prepositions, etc, for words and phrases so I would be worrying about this anyway.
Many people suggest that the search engines are all knowing of synonyms, plurals, etc. of the same word stem. I agree that it's largely true, but I have found many situations where it just is not so, and because of that, I do feel like a little variation in the usage of your keyword stem is probably a good idea.
So, my answer would be to suggest that you do actually vary the way you use your keyword, but as you do that, it's quite important, like John Conde suggested, to write your content to make sense to human beings.