Problem pronouncing 'TH'

>>I think Cockney and Cockney-influenced Estuary also have a shift of initial /ð/ to [d].<<

This shift seems to be, by far, the most common case of interdental loss, whether phonemicized or in realization, in English dialects. I would almost go to the point of saying that initial /ð/ seems relatively unstable in English overall, with it being common for dialects which otherwise consistently preserve interdentals to shift such.

I would like to give my contribution here,,,,Being myself from London but in Rio de Janeiro for all my life (just round up to get to this 'all' hehe) I must say (rather confess!) that the /TH/ sound is to me a great problem too. I can´t seem to get simple words like "thanks", "they" "there" right at all,,,, although no-one seems to mind that, neither complain...
So going into words:

My TH´s: for shortening´s sake /TH/ = /f/ or /v/, when /f/ and /v/ should be the "correct",mainstream pronounciation.

/f/ - just place the tip of my tongue right inbetween the front teeth (upper and bottom), like if I were actually biting it and say F. But my /f/ is so short and tending to a soft /t/ I sometimes ask myself what the general perception of it is like. Anyone forms /f/ like me??

/v/ - Now this is troublesome! I am sure I am overtly unable to get it right. My /v/ is a /d/ (D) with only one diference. Taking two perfect words for comparison ´day´ and ´they´ I usually pronounce them the same way but with difference in the vowel quality, just that! My D, as I can infer, is done by hitting the upper-front hard palate, just before the teeth. And if I say ´d´ (the letter) or ´thee´the result should be the same and just maybe I sometimes touch my teeth when I do the /v/ of ´thee´. But the sound I hiss is D, not V.
My ´that´is just a ´dat´.
You might want to know as well: the general perception of my accent is general Southern-London like. But, I would say, is something like an "RP with International Allowances" being for so long abroad... a wee bit from this, a little bit from that...

HOPE i HAVEN´T MADE A MESS of it ;D

Anyone else?
cheers!

Antonio: I'm confused by your post. The contrast between /v/ and /D/ is not really exemplified by THEY-DAY in the "standard" accents (RP, GA). The other contributors here have been using IPA, so /v/ is the first consonant sound in the word VAN, and /D/ is the first consonant sound in the word THAN.
So, unless you consider the "correct, mainstream pronunciation" of THEY to be /veI/ (which I doubt), I suspect you mean to write /D/ every time you write /v/.
The point here is that for many speakers, especially Londoners and speakers of Estuary English, /v/ and /D/ do not contrast, nor do /f/ and /T/. So, for these speakers, THIN and FIN would be homophones, they would both phonetically have [f].

Gabriel: sure. /v/ = /D/ now as you put it ;g

I´ll just copy&past from someone else´s post to try and explain (sorry, am a bit awkward writing with these IPA sound. Though I can read them quite well)
What I really meant was:
/f/ = /θ/ like in thin
/v/ = /ð/ like in them
/d/ = D like in day
/t/ = T like in time
/v/ = V like in vowel
So, consider the way I work my tongue and teeth settings towards those phonemes.

this /ðIs/
that /dat/
than /dan/ not /Dan/ = /ðan/
day /dA/ or /dEi/ (which is the correct character rep?)
they /dA/ or /dEi/
those /dOwz/
thin /θIn/
thread /frEd/
thank /fank/
through /θru:/

messed up huh!!?
Maybe I should record a sample file!
Hope I have made myself clear this time.

if you,native speakers have problems in pronouncing english.so, how about foreigners?

Most foreign learners of English pronounce Birthday as Birdday or Burdzay.
It's not difficult to pronounce TH (or DH) isolated, but in some consonant clusters (especially between /z/ and /s/, like in ''it's the sea'') it can get pretty tricky.

<<if you,native speakers have problems in pronouncing english.so, how about foreigners? >>

I'm a native Spanish speaker and it's easy for me to pronounce the TH sound. In fact it's one of the easiest.

Well of course, since Spanish has that phoneme.

One just needs to get used to the practice of saying the voiceless 'th' (In 'think') by putting your tongue between their teeth and breathing foward. The voiced one instead is articulated in the same way as "D" with your tongue still in between your teeth.

Talking about the "TH" sound in English (as in words like "think" and "through") it's difficult for us, as native speakers, to understand just why some (not all by any means) learners of our Language are unable to voice it properly. It's just a question of sticking your tongue outwards between your almost clenched teeth and exhaling breath. I don't think they're genetically incapable of producing this sound - it's more to do with the fact that this particular sound doesn't occur in their own native Languages. Germans are a good case in point. The "TH" in "think" becomes an "s" as in "sink", while the unvoiced "TH" sound, as in "this" and "that" becomes a "z" as in "zis" and "zat". Maybe this isn't always the case though - how would they pronounce "throne", for instance - "srone"? Mmmmm...probably. I'll have to escort a German friend round the House of Lords at the Palace of Westminster one day and ask him to name the very ornate gold seat at the far end of the chamber.

What languages do have the TH sound? Spanish , English and Icelandic are he only ones as far I know.

''Germans are a good case in point. The "TH" in "think" becomes an "s" as in "sink", while the unvoiced "TH" sound, as in "this" and "that" becomes a "z" as in "zis" and "zat". Maybe this isn't always the case though - how would they pronounce "throne", for instance - "srone"? ''

Most Germans would pronounce it as: trone,
th-->t,
dh-->t

is the most common way of pronouncing TH and DH in
Continental Europe's (World) English

''What languages do have the TH sound? Spanish , English and Icelandic are he only ones as far I know.''

In Continental Portuguese, intervocalic d is pronounced as the in ''that'', but it's an allophone of D, not a separate phoneme.
Idem in Spanish, th/dh are not separate phonemes, which can be attested by its absence in Latin American Spanish.

I refer to the th sound like in tooth, not just d like in the.

Greek also has the "th" sound, both voiced and voiceless. Small note to Damian, the "th" in "this" is the voiced version.

(''What languages do have the TH sound? Spanish , English and Icelandic are he only ones as far I know.''

In Continental Portuguese, intervocalic d is pronounced as the in ''that'', but it's an allophone of D, not a separate phoneme.
Idem in Spanish, th/dh are not separate phonemes, which can be attested by its absence in Latin American Spanish.)

"d is pronounced as the in ''that''" no it isn´t, it´s pronounced the same way as in English as the d in door.
If this phonetic sound was present in Portuguese you would never hear them pronounce them as dem, which they do.

In the case of Spanish the lisp is pronounced when ever ce or ci is present in a word and is pronounced in the same way as th in therapist and think respectively.