I agree with Abdalá. But then again, what do I know?

It’s perfectly fine to start a sentence with a conjunction, and most grammar guides permit it. The belief that it’s not okay is a result of teachers making up their own rules. Instead of teaching when it’s acceptable to start a sentence with a conjunction, they taught an overgeneralized rule. It’s just lazy teaching. See here: <a href="https://www.quickanddirtytips.com/education/grammar/can-i-start-a-sentence-with-a-conjunction" rel="noopener noreferrer" target="_blank">https://www.quickanddirtytips.com/education/grammar/can-i-start-a-sentence-with-a-conjunction</a>;

Thank you so much for your very clear explanation, Adam S.

You know what, whenver I saw people writing "but" at the begining of a sentence, I immediately told them to change it into "however". Then, there're still many people who continue using "but'' in that way, I start wondering myself if it is okay to use "but" like that...

I am still looking forward to knowing the opinions of other people on italki.com to have my own conclusion.

Thank you one more time, Adam.

This is an area where people have some different opinions. Some have been taught to never use but at the beginning of a sentence. But, when the author want to highlight the "but," it can be effective when used to start a sentence. "However" is generally more acceptable at the beginning, but since it is basically a synonym for "but," it arguably raises the same issue as "but," which is that, as you point out, it is a conjunction. That said, some might think that "however" when used at the start of a sentence is a bit awkward, but "but" is nice and crisp. This is sort of a case of some authors choosing to break the "rules" for more effective communication. The same can be said for starting a sentence with "and." It should be done sparingly no matter what, but it can be effective in some occasional instances, even though it is not really proper.

Yay! Thank you so much, Abdalá. I am reading the information from the link you send me ^^