When the political regime that rules it has it's origin in the country itself.
Generally, "constituent country" is a term applied to territories that call themselves countries, that are a part of a bigger sovereign country. For example, England is a constituent country of the United Kingdom and Greenland is a constituent country of the Kingdom of Denmark. If you mean a sovereign country within another sovereign country, that is called an enclave.
Lady Sovereign is from England.
It means that was the official day of creation of your country as a sovereign unit
North Korea thinks that it is Sovereign. But they are not.
Yes
Canada is an independent country.
China is not in any country. It is a sovereign country in its own right.
One of the many definitions of "Sovereign" is, the reigning Monarch of a country. Englands current Sovereign is Queen Elizabeth II.
None. Bulgaria is a sovereign country itself.
South Sudan is a sovereign state (country) and a member of the United Nations. Thus it depends on what you mean by a state. The term can mean an independent country, in which case the answer is yes, or a subdivision of a larger country, like Uttar Pradesh in India, in which case it's no.
It is an independent sovereign state.
Italy is a sovereign country.