There is a good article describing the definite (and other) articles at https://en.wikipedia.org/wiki/Article_(grammar) . If you look at the list of languages with definite articles, almost all come from one of two geographic regions: Polynesian, and Southern and Western IndoEuropean/Afro-Semitic. This is a contiguous area on the map despite it covering two language families and not covering all of IndoEuropean - specifically it leaves out the Slavic languages that include Bulgarian. The definite article in some languages such as Greek and Arabic is, I believe, fairly logical - you put in an article if it is definite. So you can imagine a Bulgarian learning this fairly quickly. In fact the definite article is so common in Arabic that English speakers think it is part of the noun. That is why many nouns from Arabic such as algebra, alcohol, alchemy start with al. Al is simply the definite article but Europeans thought it was part of the noun.
But there is an inner group of languages, again in a neat geographical area - Germanic and Celtic, and to some extent Romance, that have more complicated rules that are hard for anyone (including Bulgarians) to understand. You don't need an article with something definite if it is obviously definite - so no article with names - you don't say the David, and you don't need one with Western Civilization because the word Western makes it clear that we are talking about one specific civilization. Similarly instead of saying the head of the king, we say the king's head, with no article before head. I don't need to go on as there is plenty of discussion on this site about the definite article.
The main point is that the definite article in Western Europe (particularly the North West) is unusually complicated, especially for someone who doesn't have any at all.