English has two TH sounds: voiceless /θ/ as in think and voiced /ð/ as in this. Most languages have neither, so learners swap in the nearest sound they own: t, d, s, z or f.
Put the tip of your tongue lightly between your teeth and blow. No voice for /θ/; add your voice for /ð/. If your tongue stays behind your teeth, it comes out as t or s.
34% of 48,249 scored /θ/ sounds in the SayWhat app were scored red, from at least 5,805 learners.