If you want to discuss the French, the English or the Ukrainians, you are using what is called a demonym or gentilic to describe the people of a specific nation. You can further use a demonym to talk about people who live in a city, a town, or a region. For instance you could discuss San Franciscans or Parisians using demonyms, or use the general term Southerner for people living in the South part of a country.
In most cases a demonym changes the name of a country or specific geographically defined space by altering the ending or by altering the whole world. There are numerous spelling changes or suffixes which can be attached to the end of the name of a country or place in order to produce demonyms. These include the suffixes:
It isn’t always possible to tell why one suffix is more appropriate than another. The suffix ian is used for the people of Brazil, Brazilians. It could be argued that Brazilites is equally appropriate, or that Brazilish is similar to British. A resident of Bengal is a Bengali, and it’s not clear why he or she is not a Bengalian or Bengalese.
A few types of demonyms significantly change spelling of the country or place origin word. In the English language French is significantly altered from France. Residents of Denmark are Danish, and people from Liverpool are Liverpudlians. In some cases, the demonym is completely different than the word used to describe a place. It might be hard to guess that people haling from the Netherlands are the Dutch.
You can use a demonym as an adjective or noun, like Chinese or Japanese. You could refer to the Chinese, or to a Chinese restaurant. Sometimes when the term is in adjective form the suffix may change. People from Spain are Spaniards, but you might listen to Spanish music or watch a Spanish film. Referring to a single person can make a difference in word endings too. While British refers to inhabitants of Britain, Englishman/Englishwoman or Briton are more common singular forms.
Sometimes more than one demonym exists for the people of a country, like British/English. Occasionally, a demonym has been used in the past and is no longer used, and is meant to reference an older time. The modern resident from Greece is a Greek. Those who lived in Ancient Greece were Grecians. Someone who lives in Norway is Norwegian, but people still discuss the Norse gods or Norse mythology of old.
One interesting type of demonym is those for fictional places, or those describing non-existent people/aliens like Martians. Science fiction and fantasy works are filled with invented demonyms. You can hardly get through a Star Trek episode without running into some like Vulcans, Klingons, Cardassians, and Bajorans.