I am asking a voice teacher, and I’m learning that the difference between “theater” and “the” is that one of them is voiced and the other is unvoiced. “F” and “V” are similarly voiced and unvoiced, and “thistle” and “this”. I have received the following on how to make a “th” sound for a non-native speaker.
Start with a d. Then try to get them to put the tip of the tongue between their upper and lower teeth while saying d, the d will distort into an approximation of th and they can clean it up from there.