How to give marks for a sentence using c#

Without formalization of the problem, there is nothing to talk about.

Something tells me you don't have such formalization. Let's see: instead of "I want to compare user's answer with original answer" you put "i want to comapre user's anwser with or_ginal anwser". Five words out if eight are misspelled (62.5%; to me, a little too many for the one going to automate testing of a level of literacy :-)).

Do you want to reduce as many points for capitalization as for inverted letters? How about wrong letters (it indicates more of illiteracy then mechanically inverted letters, perhaps during fast typing)? What about total gibberish? How are you going to identify the words if they are misspelled and the number of works is different? There can be a correct variant different from the "original spelling". What, are you going to subtract points even if the answer is actually correct? I don't even mention syntax.

I feel you whole life won't be enough to answer all the questions. If you think I'm wrong, please provide a fully formalize system of testing. Programming won't be a problem at all. :-)

—SA