You will feel the improvement straightaway. Your continued development will depend on how often you practice and have conversations with people using the sounds you’ve learned. It will also help if you listen and watch programmes with Standard English speakers. The more you prepare and give talks or presentations in English, the better. The more you prepare and tell stories using the sounds you’ve learned, the better. Keep targeting the sounds that are the most difficult for you by using the relevant tracks on the “Speak English Clearly” CDs.
