In search engine optimization we have to write many articles for particular project. Can anyone please provide me the information how to write fresh , unique and without any grammatical errors plenty of different articles.

Read a lot of blogging sites,articles sites and other informatiive websites like mashable,forbes etc.Only by reading and gaining some knowledge of the topic it is possible to write articles.

First of all, clear your topic in mind what you write about subject.
Then research and read on those subject online at various blogs and articles.
Side by side, write your article with your own words.
Choose appropriate word that most frequenly used in your subject.

Well, there is no hard and fast rule to write a good article !! I'll say try to create some new concept before moving forward like if you write the same old thing that you think is unqiue isn't going to help you in anyway !! What would help you is having something different try thinking out of the box rather than murmuring about the same that others have written.

First choose a topic. Then write/type a draft. And then elaborate from it. There is no "how to produce a perfect article" manual.