" I know what you are thinking about" is not a compound sentence because a compound sentence contains two Independent Clauses joined by a coordinator. ( and, but , or etc...) >>> ID clause + coordinator + ID clause <<< [ ID = Independent Clause] e.g. I love you and you love me:) " I know what you are thinking about" can be a Simple Sentence or " a Complex sentence. We can say this sentence is a Simple sentence because it contains only one Independent Clause. Sentence Structure : -- I + know + what you are thinking about. Subj + VT + Obj [ noun clause] = 1 ID { Simple Sentence} Noun clauses perform the same functions in sentences that nouns do. So it can be "object" in this sentence. OR We can say this sentence is a Complex sentence because it contains only one Independent Clause and one or more dependent clauses (Dependent clauses are also called subordinate clauses and begin with noun clause markers such as: that if, whether Wh-words: how, what, when, where, which, who, whom, whose, why Wh-ever words: however, whatever, whenever, wherever, whichever, whoever, whomever Sentence Structure ---- I + know + what you are thinking about. Independent + Subordinate Clause as a noun clause - Marker = what ---------= 1 ID + 1 DC { Complex Sentence } Anyway , the type of this sentence depends on individual consideration. And I think communication is the most important when we speak.

no a compound sentence contains two independent clauses joined by a coordinator like: and, or, yet, but , so, for and nor ali was ill, so he didn't go to school yesterday. the dress was expensive, but she bought it.