I would suggest an introductory book on linguistics, in the are of syntax (maybe with semantics). There should be references that basically break English into a parse tree, much like this

Phrase = Subject, Verb, Object, (Conjunction, Phrase)*

You will likely want to take a reasonable subset of the complete list, for there are several deviations from a standard and English has some anomolies.

Be aware that if you wish to port the engine to different languages these rules will change.

Hmm.. the subject of imparitive sentences is implictly "you", so you''d have to take those sentences as if the person was talking to the computer. Same overall effect, I just have a grammar thing.

In imparitive (spelling?) sentences, the verb is always first "Look out!", "Give me that.", unless it''s a negative sentence, then look out for Do nots and Dont''s, but I doubt people would be typing in stuff like "Don''t walk on the lava" in a video game. For merely controlling the Player Character (PC) this sort of expectation of sentences is sufficient, but actually conversing with NPCs may be quite an impractical feature for a video game. But if you''re willing to tackle it you should look at the source for other language parsers to see how they split up the language.
http://ciips.ee.uwa.edu.au/~hutch/hal/
MegaHAL might have it''s source available.

You need to consider generating a ''dictionary'' for your game, of all the verbs, nouns, adverbs, adjectives, etc. This allows your parser to make a much better guess at which parts of the sentence are what. You can then strip out the fluff (articles like ''the'', or ''a'', generally), validate the sentences (certain verbs will need no object, some will need one, some also need an indirect object), and correspond that to some sort of look-up table in your game that corresponds to game logic, usually some sort of NPC knowledge base. To be able to ask questions requires a different sentence structure (and therefore slightly different parsing order) than for statements, so looking for a question as the first thing you do could possibly simplify it.

Also - adjectives and nouns don''t necessarily have to be treated separately, as they are both just variations on ways to identify that item from others. The noun specifies type, the adjective specifies appearance etc, but as long as it distinguishes that item, you have as much info as you need.

Look up recursive descent parsing somewhere - it''s more commonly used for interpreting scripting languages, but you should be able to use some of the ideas for parsing English or any other language.

There''s one thing that no-one has thought of yet - spelling. It wouldn''t look to realistic if the player said "Giv the axe to me" and the keeper said "I have no idea what your talking about". Once you have a complete dictionary, I suggest that you modify it slighly to include slightly mispelt words, but giv (sorry) them the same meaning as the correct ones.

Don''t bother about it now, include it when you have the rest of the game working.

Regarding mis-spelt words, you could use some heuristic to compare an unknown word with all the words in the dictionary and try to guess the meaning. You could cut down the search by analysing the type of word you''re expecting (verb, noun etc) and search those word lists first.
So if "Give me a sord" matches a verb phrase best then the computer knows that sord is most likely a verb, searches the list of verbs comparing word length, word ordering and word fragments against the verb dictionary and hopefully come up with the word sword as the most likely answer.
You can then use the fact that the word was mis-spelt to ask the player a question. "Did you say you wanted a sword?".
None of the above is difficult it''s just getting the mix right.

Also, it would be useful to have many different examples of each phrase type, from
"I would like a sword"
to
"Ug want sword"
As long as these didn''t conflict or confuse other phrases.

Mike

Another thing to remember: most people like to use pronouns in communication. They are easier to type than always using the proper, longer, noun. Also, your NPC needs to remember the things that were said to it. At least for a short time. I had an old text adventure game that used natural language very well. It understood context when it came to individual words. Some words could be both a verb or a noun. An example is the word "check." The check is in the mail. And, I will check the mail. The game understood the difference between these two sentences. Any dictionary you make would have to include how the word can be used as well as the definition for each use.

Making friends one burger at a time.