If something is common, it’s ordinary, regular, normal, or popular.
A common form of transportation is the car.
A common form of communication is the phone.
Road construction is a common problem to deal with in the summer.
It’s common to say hello upon greeting another person.
Most people suffer from the common cold at one time or another.
A basic understanding of the physical world is now common knowledge among most educated people.
"Bob" is a very common name in the United States.
Television is a common form of entertainment.
At first, the internet was not a widely-accepted form of communication for people, but now it’s commonplace. (commonplace = average; ordinary. Note that "commonplace" is one word)
They make a good couple because they have a lot in common.
The word "commonly" is an adverb.
Most people commonly say hello upon greeting another person.
Salt and pepper are commonly used in flavoring food.
The idea of an omniscient (all-knowing) God is a commonly held belief.
Marriage is a commonly accepted practice around the world.
Note: When two people share the same beliefs and interests, we say they have a lot in common.
