The usage of the and a/an

 here
The above site is very authoritative. Look at Table 3 for decision. Remember the 5 sources
of definiteness. Should read it many times to memorize.

Here is a summary of the article:

• Understand 2 concepts: countable/uncountable, definite/indefinite
• Somehow remember Table 1: Some common uncountable nouns
• Definiteness: known to both the speaker and the listener.
• 5 sources of definiteness: previously mentioned, a ranking adjective, a unique thing, modifying structures, context
• Table 3 is the key: singular countable noun: use a/an for indefinite, use the for definite; plural/uncountable noun: use none for indefinite, use the for definite.
• 'the" is not used with most singular proper nouns (names of things). There are exceptions. Need to memorize.
• "the" is always used with plural proper nouns.
• Which of a/an to use depends on pronunciation, not on spelling.

Special cases:

1.  Sometimes countable nouns are considered to be uncountable and used without an article. This happens quite often with places/institutions (proper nouns).
2.  Do not use an article when you are referring to tables, figures pages, appendices, chapters or sections in your written work.
3.  Do not use an article with a noun after an '-s' possessive.
4.  Articles are not used with days and months, unless you are talking about a particular day or month
5.  Note the following when talking about television, radio, cinema and the theatre.
6.  Do not use an article with television when you are referring to the medium.
7.  Do use an article, however, if you are referring to a television set
8.  The is not used with titles which include the name of the person
9.  We usually uses possessives rather than the when we talk about parts of the body

It claims:
Keep in mind that native speakers of English seldom use articles incorrectly; therefore, any errors that you make are very noticeable and distracting to them. That is why you should make an effort to use articles correctly.

If you search "article usage" here, you get a lot of resuts. Reading them can be educational. I am not alone to be confused.

If so confusing, I wonder why 'the' exists in English in the first place.

Here  (the first post), even an English teacher is confused.

Hereis a post with 10 rules of THE usage.

Another post (the 4th post) about the idiom source of the usage.