How to check English grammar in C# form application

Look here for Open Source:

http://www.thefreecountry.com/sourcecode/spellcheckers.shtml[^].

Also here:
http://www.thefreecountry.com/utilities/free-spell-checkers.shtml[^]

Also Google something like that:

open source spell checker windows

This days, Open Source collected more resources, you have some good options for development.

You've picked a tough subject! Spell checking is a trivial exercise that has been done over and over again, but I've never heard of an effective, accurate grammar checking tool for the English language. Most native speakers do it badly, and their writing is far worse - it's a very complex language. The rules of English grammar are complex and many, and there are a lot of exceptions that have fallen into general usage of the years.

I would attack this with a rule-based inference engine, and populate the rule base with the help of someone truly expert in the formal language. In fact, I would preprocess the input with a diction checker built on similar functionality, to cull out the many misused words that most people think are correct. This would have to analyze word groups to attempt to ferret out the intent of the writer and resolve errors of usage - their, there, they're; its, it's; too, to, two - and so on.

Your engine would have to add a learning function, as well, to accomodate new additions to its rule base as new situations are encountered. It's a nasty problem, but one that needs to be solved by someone. That someone might well be you! :-D

There is a wealth of articles, here in CP. Take a look[^]