Is the word nations singular or plural? can you explain with an example?

The word "nations" is both singular and plural. In the context of "The United Nations", it is singular. There is only one organisation known as The United Nations. In the context of "all the nations", the word "nations" is plural. 
Examples of sentences using the word "nations" in a singular and plural context are as follows:
The United Nations is an intergovernmental organization that consists of representatives from its member nations. 
The first "Nations" is singular because the sentence is speaking about the United Nations. The second time the word nations is used, the word is plural because the sentence is talking about many countries or nations that make up the United Nations' membership.