In words like bottle and mitten, the "t" really isn't a "t" sound; it is more of a "d" sound or a very fast "t" sound. Practice the "t" sound with the words button, carton, brighten, tighten, fatten, eaten, rotten, matter, butter, flutter, water, bottle, settle, and metal. In American English, the "t" sound is very difficult to hear in some words. An example of this is the word butter, where the "t" sounds more like a "d." Remember, the way people speak English in the United States is different...
