If you said 'You are da best thing' we would understand you, but it would sound strange. It would be clear that you were making the wrong sound - we would hear that you were saying 'd' instead of 'the'. When you say 'd', the tip of your tongue is touching the top of your mouth. All you need to do is move the tip of your tongue a little bit further forward, so that it is touching just behind your top teeth. If you make the sound with your tongue in that position it will be a correct 'th' sound. It's a very light, quick sound - the same as a 'd' but with the tongue behind the teeth. That's all you need to do. Try it and see!

for example: "you are THe best THing, THat´s ever been mine" "I THought THat I heard you laughing" I know that if I speak slow, I can say the sound "th" good but when I´m trying to talk fluently, can I change the sound "th" for d"???? like this: "you are Da best THing, Dat´s ever been mine" "I THought Dat I heard you laughing" can you get it?? I repeat that I´m talking about speaking not writing

Can you give some examples, please? Which sounds?