de
The character 的 is used in three main ways. The first is to indicate possession (somebody's something). For example, to say "father's brother." you would write 爸爸的弟弟.
When talking about family relationships, this use of 的 is optional. When you are indicating the possession of another type of object/thing, the use of 的 is mandatory.
You could say 爸爸弟弟 or 爸爸的弟弟 (father's brother) but you must say 我的书 (my book).
If you've ever taken Spanish, you'll want to take care not to confuse this with the de in Spanish that also indicates possession. If I say El libro de Paolo it means "Paolo's book." or "The book of Paolo." Note that the main difference between the Chinese and Spanish example is the order of what possesses what.
The second way this character is used is as part of a adjective clause to. To say "this book is good" you could say either 这本书很好 or 这本好的书. In this usage, the 的 comes after the adjective and before the noun it describes.
The final way we use 的 is to indicate emphasis or agreement/intent to do something. Such as 我买东西的. (I am buying stuff.)
