The Russian language, like many languages, doesn’t have the th sound. This sound is difficult to learn for foreign speakers.
Once my student has learned the demonstrative adjectives we begin th practice.
There are two forms of this sound, voiced /ð/ and unvoiced /θ/.
Both sounds are made by sticking your tongue out and between your teeth.
three, thank you, and Thursday are unvoiced
The sound is air moving between your top teeth and tongue. Don’t be lazy and make a /f/ or /s/ sound.
this, that, these, and those are voiced
Air is pushed between the gap between the top teeth and tongue. Don’t be lazy and make a /z/ sound.
Repeat, repeat, repeat
Repeat the demonstrative adjective words.
Repeat while standing in front of a mirror.
Repeat while recording ourselves, and then playing it back.
Repeat the th sounds from various lists of words. For example…
This sound alone is a major reason why foreign speakers struggle to speak without an accent.
Awesome English 
Very useful and clearly explained lesson! Thank you, Laura!