Another thing I have noticed about English (as a native speaker) that many people (even native speakers) get wrong on written English:
With nouns, possessive is formed by adding "'s" (with an apostrophe), so "Sarah" becomes "Sarah's" and "dragon" becomes "dragons". Sounds simple, right? (Well, there *is* the special case of plural nouns ending in 's', and I am not sure about singular nouns ending in 's'.)
With pronouns, however, possessives do *not* contain an apostrophe. Often, the form is irregular, so "she" becomes "her" (but note that pronouns often change when used as the object of a sentence). On the other hand, we have "it" becoming "its", which some people confuse for "it's" (which is wrong).
The think is, however, that "it's" is an actual English word; it is a contraction that expands to "it is". Therefore, a spellchecker will not notice the mistake.
I have been noticing this mistake a lot lately, and it is irritating. Remember, "it's" = "it is", and if you want to show ownership, the word you want is "its".
(Note that this mistake shows up only in written English, as both "its" and "it's" sound the same when spoken.)