国 vs 国家（近义词）

1)国 is a quasi-agglutinative word and para-measure word.国家 is a free word and can be used independently:
2)国 cannot be used instead in examples①②.“国”用于国名，如“中华人民共和国”、“美国”、“日本国”等。另外，前面加数词，可说“一国两制”、“两国建立了外交关系”等
3)国 is used in the name of a nation,e.g.中华人民共和国，美国，日本国，etc.A numeral can be used in front of it without a measure word,e.g.一国两制(one country,two systems)、两国建立了外交关系(The two countries have established diplomatic relationship),etc.
4)A noun of locality can be added to 国，e.g.国外(abroad),国内(home),etc.It can be preceded with a demonstrative pronoun or personal pronoun,e.g.我国(our country),各国(all countries),你是哪国人(Where are you from?),etc. What's more,it can be preceded with a verb,e.g.出国(go abroad), 回国(return to one's country),复国(restore the ruling),亡国(doom of a country),(科教)兴国(in vigorate a country by science and technologies),etc.Otherwise,国 cannot be used independently.