Simple -- You spelled everything right, and you didn't break any recognizable English grammar rules (at least according to Word). :)
Word doesn't/can't actually read your document, or know your intent, and it's not an English professor. So just because the order of words, and/or incorrect tenses (etc.) you used may be confusing to an English speaker, doesn't mean that it's grammatically incorrect (at least not according to Word's limited abilities with the English language).
Here's a site that generates "sentences" made from 'random' English words: http://watchout4snakes.com/wo4snakes/random/randomsentence.
I generated 5 different sentences, none of which make any real sense when read, and none of which were recognized by Word (2013) as having grammar or spelling problems, because technically, they don't:
- Can a banal fountain boggle within my territory?
- The thief cooks the college without the padded ownership.
- The presence forks the pompous secretary.
- The imperfect custom fools the friendship.
- The stagger retains the spare trolley past its technique.
Never trust a word processor to be perfect at spelling detection, and even less-so when it comes to grammar. Instead, get someone to proof-read it for you.