Do natives sometimes pronounce TH as D?

Most of the other commenters are going to reply no, but it’s actually pretty common to accidentally pronounce “th” as a stop after other stops, as in “keep the change” “grab that” “take this.” It’s one of those things native speakers don’t notice unless it’s done in the “wrong” place, lending to the perception that it’s only ever done in certain accents.

Unfortunately, it’s likely just that you’re not used to having to distinguish these sounds. While some natives do actually not have dental fricatives, this is dialectal and most speakers make a clear distinction. It is quite common in certain positions, though, for natives to turn the fricative into a plosive. But it’s a dental plosive, not an alveolar one (which is what English /d/ is). They’re still contrasted; you just don’t end up with the hissing quality of a fricative sometimes. This can make it harder to distinguish, but to the majority of native speakers, there is a clear difference between the /ð/ and /d/ because they play a role in distinguishing many words: duh - the dare - there day - they den - then Etc. It may be worth spending time listening to and trying to repeat minimal pairs in order to get more comfortable with the difference. As natives, we have had many, many years of constant immersion and exposure to these sounds so our ears are trained to pick up on what, to other people, may sound the same. It just means their ears are not trained to listen for the difference, but this can be helped with practice and time.

Generally, no. It is called th-stopping and is usually associated with countries where English is a commonly used but not a native language.

Others already covered everything well, but I'd just like to point out that this is something to specifically avoid. In my dialect, it's really common to do that in baby talk, and one of the things people really latch onto if they notice it in their subconscious language judging. If someone were to use a voiceless alveolar fricative (the 'th' in 'with') where it should be voiced ('there'), I feel like you'd be judged as an Educated Foreigner because we seem to be biased in favor of Germanic-speaking countries, but if you go to a stop ("d"), then you "can't speak English" even if that's the only mistake in your Master's thesis. I half hope I get downvotes for this because it's prejudiced trash, but commenting anyway in hope it helps.

are you asking if native speakers do one of the most common things in the english language

Although some regional dialects do it, I think it’s more likely that what is actually happening in your case is that the people you’re hearing are pronouncing the sound normally, but you have trouble hearing it correctly because it doesn’t exist in your native language.

Learn British English instead.

Sim, o pessoal ta dando uma explicação meio estranha, mas é pq o som de D no português é um pouco diferente do D em inglês. É normal usar o som de D, na real ninguém usa o som de T, TH praticamente sempre é falado como o D do português,

Yes, it’s called th-stopping. It’s nonstandard but common in many dialects in the United States, particularly AAVE, Great Lakes dialects, and in the Northeast (especially NYC). I believe Irish English, Scottish English, Caribbean English and Indian English also frequently use th-stopping. The exact details of how this works varies. Sometimes the *th* sounds are pronounced as alveolar stops [t d] (the same sound as *tin, dinner*), and sometimes they are pronounced as dental stops (like [t d] but with the tongue in between the teeth). However, it’s not standard and often associated with working-class speech. It’s not limited to that — I speak a fairly standard American English and am not really culturally working-class but I use th-stopping sometimes — but that’s definitely the association Th-fronting is a related thing where *th* is pronounced as /f/ or /v/, like *baf* for *bath* or *wivout* for *without.* This is common in England, and also occurs in some varieties of AAVE

It’s common in some dialects but not standard

It's dialectal

I expect most of the responses here are taking your post too literally. The hard initial "d" --as in "da Bears" with a stereotypical Chicago accent-- is indeed pretty rare. But I imagine that's not what you're observing.  More likely you're hearing the flattening of initial "th" to something more like "d" but not as pronounced as "da". Just as short "e" often becomes "uh" when one speaks fast. I bet you don't notice the "d" sound as much when the preceding sound was a vowel, right?

It can be done as improper pronunciation. Sometimes people pronounce ‘TH’ as ‘T’ or ‘F’ as well, also improper

You're probably not hearing a full \[t\] or \[d\] sound. Some dialects do use \[d\] but that is a very clear \[d\]. I think you're hearing a middle ground which is basically bailing from the full sound but is not a full \[d\] or even the sound I actually make when I bail on pronouncing \[d\] clearly. From just saying a bunch of words to myself with ds and ts, I think the /th/ sound I make is pretty close to a \[d\], but you use less tongue and it's voiced quieter? Hard to evaluate. But there's definitely a difference between what you're hearing, a "full" d, or even a half bailed "d"

No, I’ve never made it a “D” sound.

How is anyone supposed to learn English with these answers? Seriously how many tweed jackets are in the room right now? Yes, native speakers sometimes pronounce th as d. Shakin' dat ass? Come on, we even spell it with a D! That may be a dialect, but it's a dialect that is _sometimes_ adopted by a lot of people.

The Welsh do it the other way around DD is pronounced Th

Some people naturally struggle to make the sound, irrespective of dialect/accent. My brother is one of those people. Outside of that, there are some dialects where people deliberately pronounce it as a 'd', like AAVE (not all speakers but a common trait). So yes, it's normal.

I think this comes from "th" words were once spelled with the Scandinavian Letter "*ð*" during the Danelaw Period of Central Great Britain which some people confuse for a "d".