English has two different consonants that could be called "the th sound". They are both what we call dental fricatives, meaning that they are produced via the friction produced through the flow of air from the mouth through the gap that is created when you gentlt touch your tongue against your upper front teeth. But there's two different sounds: first of all, there's the sound in "this," "that," and "the," written in IPA as /ð/, which is the voiced dental fricative. Secondly, there's the sound in "thing," and "think" among others, written in IPA as /θ/, the voiceless dental fricative.
Voiced/voiceless refers to simply whether or not the vocal folds vibrate when producing the sound. You can try it yourself! Say the word "this" but hold the initial consonant and softly touch your neck right under your chin with two of your fingers. You should feel a vibration. Now do the same with "thing." You should not feel a vibration. That's voicing. The sounds /d/ and /t/ also represent a voiced/voiceless pair. They're both alveolar stops, but /d/ is voiced and /t/ is voiceless.
Anyway, now that we've got that out of the way a bit more discussion about how letters are not sounds, cause English has a third sound which to some people (myself included) could qualify as a "th" sound. I'm talking about word-initial t when it's followed by a vowel.
See okay, my first language is Finnish so my first language doesn't have dental fricatives to begin with. Finnish also has very shallow orthography compared to English: you can generally expect one letter to always represent the same sound in a given environment with very few exceptions. Because of this, when I look at a word that begins with the letters "th" I'm like "that could also represent the /t/ sound followed by a little /h/ sound." Which is a sound that exists in English. Because English has a weird rule where in most standard Englishes, word initial voiceless consonants become aspirated when followed by a vowel. Compare "pin" and "bin". The initial consonant in "pin" has a little bit of an /h/ sound following it, because once again /p/ is voiceless. Aspiration can be marked in many different ways in IPA but the way I'm most familiar with is a small superscript h, like so: /tʰ/
This is what I would call the third "th" sound in English, even though to call it that would be essentially meaningless because letters are not sounds and most English speakers wouldn't even consider it a th sound. It's just a t sound, the aspiration is just one of those unwritten rules (and if you don't do it to some people you sound foreign or like you have a weird accent).
By the way, none of this is ever taught to us English as a second language speakers. We just gotta intuit it and then get laughed at when we mix up the initial sounds in "thy," "thigh," and "Thai"
That last example was an illustration of the fact that the two dental fricatives are actually distinct phonemes and not just allophones of the same phoneme. To briefly explain what I mean by this: in English, the sounds [t] and [tʰ] are not separate phonemes, but allophones of the phoneme /t/. What this basically means is that they're both considered to be examples of /t/ but there's just a rule that sometimes it gets aspirated. Because they are allophones they can not produce minimal pairs, i.e. there is no semantic difference produced if you accidentally forget to aspirate the initial /t/ in "tin". And in fact, in some dialects of English, aspiration isn't even a feature.
Because the words "thy," and "thigh" only differ in terms of the initial consonant but produce completely different meanings, we can safely say that /ð/ and /θ/ are different phonemes and not just different allophones of the same phoneme. For another example, consider "ether" and "either," another minimal pair distinguished only by the voicing of the dental fricative.
In certain dialects of English, particularly varieties of African American Vernacular English (AAVE) there's a feature where word initial dental fricatives sometimes get replaced with stop sounds. Specifically, with the voiced alveolar stop, /d/. Think of "in da club," "dis and dat," and so on. But get this: it only happens with the voiced dental fricative, /ð/. To my knowledge, there are no instances where in AAVE "think" because "dink". Now, the reason for this isn't that difficult to grasp: when these types of changes tend to happen they take the path of least resistance. /ð/ to /d/ is a change in place of articulation (dental vs alveolar, although the distance isn't that big) and manner of articulation (fricative vs stop) but there is consistency in the fact that they are voiced. "Think" would not become "dink," because a similar path of least resistance would result in "tink."
For that we need to look outside of AAVE and look to Ireland. Because in certain varieties of Irish English, particularly Western and South-Western Irish Englishes, /θ/ as in "thing" or "think" does become realized as /tʰ/. Because even when the word initial fricative does become a stop sound, it still tends to obey the rule for voiceless stop sounds where they become aspirated.
(And as it happens, as I was researching where this sound change happens specifically in Ireland, I discovered that apparently in these same varieties of Irish English /ð/ does tend to become /d/ which I have not heard before but now want to research further.)
Now, obviously these are assuredly not the only varieties of English where dental fricatives get changes to other sounds. English is already quite weird in the sense that it has two dental fricatives (which is why you get the French and German stereotype of them saying "zat" and "zis" for "that" and "this": the voiced dental fricative doesn't exist in French or German, so first language spekaers of those languages tend to approximate with the closest voiced fricative they have!) so many contact varieties of English and English-based creoles undoubtedly do similar things. There's a lot of variety out there in terms of Englishes.
Anyway I don't have any big overarching point here, besides the thing that actually started this whole thing: me and my partner talking about how it'd be nice if second language teaching included teaching just enough phonetics and phonology so you could explain and illustrate these concepts, because as I said none of this shit was ever taught to me in English class and while this is all intuitive to English as a first language speakers most of them don't have the terminology to explain these things to ESL learners.
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