I'll take a stab at your questions. I will say that the word "the" is usually not needed in a sentence and can be replaced with other words. For example "Everyone needs a good work, life balance". No one ever says "I need to balance the work and the life" mainly because that's not the expression. Your first sentence is "I need to attend the classes held in the semester" neither of the "the's" make sense. This should say something like "I need to attend classes this semester or I have to attend these classes this semester". English has two articles: the and a/an. The is used to refer to specific or particular nouns; a/an is used to modify non-specific or non-particular nouns. the = definite article a/an = indefinite article For example, if I say, "Let's read the book," I mean a specific book. If I say, "Let's read a book," I mean any book rather than a specific book. Here's another way to explain it: The is used to refer to a specific or particular member of a group. For example, "I just saw the most popular movie of the year." There are many movies, but only one particular movie is the most popular. Therefore, we use the. The site below should provide even more context https://owl.english.purdue.edu/owl/resource/540/01/

I'll just answer your specific examples. You can of course look up "zero article" to revise the rules. 1) You're talking about a specific set of classes (= use "the") in a semester that is clear to both you and the listener (= use "the"). Don't feel that you should omit "the" if you see it too often - this is a mistake. 2) You're talking in general. You cannot ever use "the" when talking in general.