Lo (pl. gli) is used before masculine nouns beginning with s + consonant (stvali) or z (zoo).
There are a few different ways to say "the." In this case, we focus on the masculine versions "il" and "lo." You use "lo" when your subject starts with a "z" (per esempio: lo zoo, lo zucchero) OR when it starts with "s + consonantsp, sc, st (per esempio: lo specchio, lo state). You use "il" with subjects that don't have "z" or "s+consonant" (per esempio: il bambino, il gatto). When you use "lo," the plural version translates to "gli." When you use "il," the plural version translates to "i." So, because "stivali" starts with an s+consonant, you would assume before it was plural it was "lo", which translates to "gli." Gli stivale Examples: lo specchio, gli specchi//lo zoo, gli zoo il bambino, i bambini//il gatto, i gatti
All nouns (and adjectives) that in the singular end in "e" (masculine or feminine), form their plural ending with an "i".
lo stivale > gli stivali (boots)
la chiave > le chiavi (keys)
il fiume > i fiumi (rivers)
veloce > veloci (fast)
I went to forvo.com and it seems to me that in the pronunciation of gli, the "gl" sound is made as if making the L sound except touching the tongue to the upper side teeth instead of the front teeth. Unfortunately, they do not have the phrase "gli stivali", but you will find both words there separately and you might see some similar combinations.
You say "gli stivali." Many people complain that the audio and pronunciation is horrible, and often it is. But this is a computer generated voice, and so it can't convey as much emotion, enunciate as clearly, and generally be understandable. Also, much like English, Italian natives speak very fast, often slurring words. I'm on mobile, and I'm not sure if on computer it's exclusively sound, but I have text and i just sound it out, and along with the voice, I'm able to create a pretty good guess of how it's supposed to sound.
Haha, yep, the yistivali... wait, what's yistivali? Thats not a word at all, italian or english, and what does it have to do with a comment about abundant organic life in boots that Duolingo seems so obsessed about that in kind the person who wrote the original comment that you are currently replying to the person who replied to the original one wrote a comment intended to be taken humorously and it was as at the time of this comment's writing it has 22 upvotes and I myself chuckled when I read it as I was well aware of the common reference of Duolingo to the aforementioned abundant organic life that takes a liking to the smelly and humid atmosphere of a boot in anyway whatsoever? I have forgotten the point I was trying to prove when I first began to write this reply to your strange reply to a somewhat sensible reply to a comment that was intended to be a humorous reference to Duolingo's apparent liking of boots and organic creatures. This was a waste of 5 minutes of my life but I feel strangely accomplished. That is all. Goodbye.
I created a comment to clear up an error in the comment I am replying to, but deleted it accidentally. While creating a comment to explain the incident and explain what the error was and what it should be, I deleted that as well. Hurrah. That was sarcastic. So I will very shortly summarize. The comment I am replying to stated that the comment it was replying to was a reply to a reply, but it was instead a reply to the original comment about boots and snakes. See? Summarized. That is all. Goodbye.
It turns out that I did not delete any of the comments, but successfully posted them all. Therefore I wasted HALF AN HOUR of my time writing comments explaining the nonexistent problem of my deleted comments when it should have taken half the time. They simply did not show up on my screen and I am thoroughly annoyed. That is all. Goodbye.
I wrote a comment of similar length to the one I am replying to in order to clear up an error, but accidentally deleted it. Hurrah. That was sarcastic. I will address that error instead in this comment which is a shortened version of the one I deleted, which sums up to this. The comment that I am replying to, my own, stated the comment I was replying to was a reply to a reply of the original, however it was simply a reply to the original. See? A much shortened version. That is all. Goodbye.
And now I re-read my comment for grammatical, spelling, and factual errors that there are bound to be, and indeed I realize that your comment was not a reply to a reply, but one to the original comment about the snakes and boots and whatnot. I apologize, but then I am led to question Duolingo's choice not to include an edit option on comments that I would have utilized if there was, and you also may question if it is a choice that they have and I reply yes, it is simple of them to include an edit button which the result of tapping or clicking will be the ability to do what the button reads, edit, one's comment. I believe that this comment finally clears up any errors I made in the original reply to the reply to the original comment about boots and animals, and hopefully also does not produce any further errors that I must correct. That is all. Goodbye.
"Gli" is used in plural masculine forms, when
a) singular article is "lo", which happens most commonly when the word begins with s+consonant (lo stivale, gli stivali). Also when it begins with z, y, ps, gn (at least in most cases).
b) word begins with a vowel, so the singular article is contracted to l' (l'amico, gli amici)
They are masculine. "Lo" is masc. sing. and "gli" is masc. pl. They are used with masculine nouns that begin with s + consonant, gn, pn, ps, x, y, z.
"Gli" is also used with plural masculine nouns that begin with vowels. L' is used with singular masculine, and feminine, nouns that begin with vowels.