The t/d thing is not at all like l/r in Japanese
The dental plosives represented by the letters <t> and <d> in English are differentiated mainly by the fact that the <d> is voiced and the <t> is unvoiced, that is, the vocal chords are vibrating during production of the <d> but not during the production of the <t>. A better example for demonstrating the difference would be with a sibilant pair such as <s> and <z>. Hold your fingers to your Adam's (or Eve's) apple and say, alternately "sssssssss" and "zzzzzzzzz". Notice that there is a vibration in your throat when you say the "zzzzzzzzz" that's not there when you say "sssssssss". The same thing is happening when you pronounce a <d> and a <t>. This is voicing When the sound is at the beginning of a word there will also be a little puff of air following a <t> that is not there following a <d>. Hold your hand in front of your mouth with the palm close to your lips. Say "town" and "down". Notice the puff of air at the beginning of the word "town" that's not there at the beginning of the word "down". This is aspiration. In English when a plosive comes at the beginning of a word if it is voiced it will also be unaspirated and, if unvoiced, it will be aspirated. In other languages this is not the case. The <t> of Tao is unvoiced and unaspirated. The <t> of Tai-chi (sometimes spelled T'ai-chi) is unvoiced and aspirated. The T' represents that sound. The English trained ear, not used to these differentiations hears the <t> of Tao as a <d>.
