My native language is Korean. And including me, probably most Korean people often confuse /θ/ with /s/, and /ð/ with /d/. The problem is, I don't know enough how sibilant fricatives and non-sibilant fricatives are discriminated. I know that they are discriminated by whether the airflow goes between the teeth or not, but such difference are made by the tongue, isn't it? So how different is the tongue?
With enough practice to speak English, I can now speak /θ/ and /ð/ properly, but I still don't get /ɹ̝/, /t͜θ/, etc.