How, why, where, and when was the Panama Canal built? How does this change any relations to the United States, and how did it change trade and commerce?
The answer to your question is in your answer, at least in part. The panama canal is in the country of Panama in central America. It was built from 1904-1914. The primary reason for building this canal was for ships to pass from the Atlantic to Pacific Oceans. Before this canal, ships would have to go around Cape Horn in the southern most tip of South America.
In terms of importance, here is an example of time and energy that is saved. If a ship wanted to get from New York to San Francisco, it would take approximately 14,000 miles, but if it went through the canal, it would less that 6,000 miles. So, you can see the sheer importance for trade.
In terms of usage, it is one of the most used canal in the world today. The sheer volume in absolutely amazing. And even today, most merchandise passes through ships on containers. It's importance cannot be underestimated.