When making the /t/ sound, the tip of your tongue should be behind your upper teeth. Your tongue should be tense, and you shouldn’t be able to breathe right before making the sound.
When making the /th/ sound, the tip of the tongue sticks out briefly a tiny bit and is pressed against the upper front teeth, so that it vibrates slightly as the air is released around the sides of your tongue and teeth. Sometimes the vocal chords are voiced when making this sound.
Here’s another trick for helping with /th/: Have your student put their index finger in front of their teeth and say a word with initial “s”. The teeth remain closed and the tongue doesn’t touch the finger. When pronouncing the “th”, the student must open their mouth slightly and then their tongue touches their finger.
Read these words to your student(s) and have them repeat.
For more pronunciation exercises, come in to pick up a copy of Pronunciation Fun! (Free for IC teachers and students.)