The United States Patent Office (USPTO) grants patents to inventions that meet three main criteria. The invention must be novel, nonobvious, and useful.
To be novel, an invention must be substantially different from anything else that is public knowledge. Public knowledge includes anything that has been previously patented, anything that has been written about in a publication, or anything that is already being sold in the open market. The scope of this public knowledge extends worldwide. So if your invention has already been patented in Europe but not necessarily in the United States, your invention is not novel and does not qualify for a patent. However, an invention can be novel even if it simply combines two existing ideas. A shoe with an air conditioning unit, powered by the motion of walking, combines the existing invention of the shoe and the air conditioner. However, the final product is substantially different and putting the two together is non-obvious.
To be non-obvious, the invention must not be easily perceived by a person of expertise in that invention’s particular field. One would be unsuccessful in trying to patent a toaster that can toast ten pieces of bread at a time because it takes an existing invention and simply makes its capacity larger. However, Jerome Lemelson was granted a patent when he thought of combining a video camera and a tape recorder to create a camcorder.
To be useful, an invention must work and serve some type of purpose. You would not be able to patent a time machine unless you could get it to work. You also would not be able to patent a new drug if it had no effects.
After you submit your patent application to the USPTO, it is the job of the patent examiner to determine if your invention meets the three requirements.
Do not immediately assume your invention cannot receive a patent if it is not clearly novel, nonobvious, or useful. What is novel, nonobvious, and useful to you may not have the same meaning to someone else, or for a patent examiner. A patent professional can analyze your invention from the perspective of a patent examiner. Furthermore, there are ways to structure a patent application differently to make your invention fit the three criteria better. Only an experienced patent professional can give you an idea of patentability.