Pronunciation is one of the most difficult aspects of English to improve. We usually have bad habits that have been in place for years if not decades. It’s hard to change these habits and requires a lot of dedication, time, and practice. For this reason, many non-native speakers do not make a lot of progress in this area.
On top of that, pronunciation is mostly mechanical. Maybe physical is a better word. This means that to produce the sounds properly, you have to be doing the right thing physically with your mouth, jaw, and tongue. Most pronunciation training involves re-learning how to execute these muscular movements correctly.
For example, for the /th/, you have to place your tongue lightly between your teeth and produce a breath sound. Non-native speakers who mispronounce /th/ usually keep their tongue behind the teeth. As a result, the sound is either an /s/ or a /d/. (Making it even trickier, is that you probably don’t hear the sound the same way a native speaker hears it!)
So considering these difficulties, there’s one thing you can do right now that’s almost guaranteed to improve your pronunciation.
Most international students and professionals simply talk too fast. Good effective native speakers of English usually speak more slowly than you’d expect. Listen to podcast host Joe Rogan sometime. He speaks very slowly and intentionally.
Listen to him talk about sharks at 2:37 here. https://www.youtube.com/watch?v=6QY1aAvH4FE
He is very easy to understand. And he doesn’t have any pronunciation errors. Even if your pronunciation, is almost perfect, you should keep it at this speed. The faster you talk, the harder it is to understand you.
As I tell myself students. Conversation is a comfortable relaxed drive on a Sunday afternoon in a peaceful neighborhood. You shouldn’t be going 85 mph. 25-30 is the usual speed limit. 😉