Complex sentence: The American Petroleum Institute, commonly referred to as API, is the main U.S trade association for the oil and natural gas industry, representing about 400 corporations involved in production, refinement, distribution, and many other aspects of the petroleum industry.
Split into simpler sentences: The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry. It claims to representi about 400 corporations involved in production, refinement, distribution, and many other aspects of the petroleum industry.