No wonder you've got some trouble with the article usage, simply because in English we need to take into account that there're many exceptions to the grammatical rules and regarding the article, we could put the following examples:
You write the article the for a group of islands (The Canary Islands) and you don't write it for some countries (Spain). Exception: Polynesia is considered to be a country by itself, so no article there; unless we'd like to refer to individual islands within its country. Other exceptions are countries like the USA or the UK. As you can see now, be careful with rules in English, 'cos there're many more exceptions. For more information, please click on here.
Still obsessed with Grammar RULES, please have a look at this teacher, talking about the Present Perfect in English:
For further practice on the article and the topic of Tourism, please listen to Anna talking about The Alps. (Handout, key & script. Audio)
On the other hand, let us now consider the matter of GRAMMAR from the opposite point of view, and think about part of Luke's podcast comments as follows: "James is a normal educated professional Londoner, but as a native speaker, I wanted to ask him some questions about Grammar, because often learners of English are very surprised that native speakers don’t really know anything about the RULES of Grammar, even though they speak the language perfectly.” For the complete Luke’s podcast about grammar, please feel free to enjoy his useful clarifications by playing the whole podcast below: