The voiceless th, like in “thin”, and the voiced th, like in “that”, are hard enough to pronounce.
When linking them to other words in sentences, it can be even more difficult! Consider: "this day", "that dog", or "with several". Saying these can be a nightmare for learners, and sometimes even native speakers!
A key technique to help with both th sounds is to have the tip of your tongue touch your top tooth—don’t keep your tongue in-between your teeth! The reason you want to raise your tongue higher is it will make transitions to other sounds, like s or z, easier.
Keeping your tongue between your teeth works fine when words appear by themselves. However, it can become much more difficult to transition when talking in complete sentences.
When trying to pronounce pairs of words with th, d, s, z, and similar sounds, another key is holding each consonant long enough. For example, when saying “whose this”, hold that /z/ sound on “whose” a little longer, then move onto the th.
Get more practice and tips in our full video: https://youtu.be/cNc3qMriKeU