‘t' is a sound that is present in many languages. However, for the Japanese language, ‘t' can be mistaken for ‘d' if the voice box in your throat is vibrating when you feel it . You can tell when it is voicing by placing your fingers in the area of the larynx as you pronounce the sound. If you do not feel any vibration, then it is ‘t'. In other words, vocal chord vibration or its absence makes a difference in the correct interpretation of the sounds produced which determine meaning. (For details, please refer to the theory section.)
However, in some languages with regard to whether the speaker releases an extra puff of air or not when pronouncing ‘t' makes a difference in meaning. For learners whose mother tongues have such characteristics, they tend to pronounce da, de, do ( だ、で、ど ) as t-sounds without releasing a puff of air and are heard like the Japanese sound ‘t'. When the learners do this, Japanese will recognize those sounds as ‘ta, te, to' ( た、て、と ) instead of da,de,do ( だ、で、ど ） . ( ダイガク daigaku (college, university)→ タイガク taigaku ).
Moreover, there are some languages in which the presence or absence of vibration is determined by the position of a sound in a word, such as ‘t' at the beginning of a word and ‘d' in the middle of a word. For speakers of such languages, although they are able to pronounce da, de, do ( だ、で、ど） ; however, when these sounds appear in the middle of a word, there are incorrect tendencies to pronounce them as ta, te, to ( た、て、と ) if they appear at the beginning of a word. ( ダイジョーブ daijōbu (alright)→ タイジョーブ taijōbu ). Furthermore, there are incorrect tendencies to pronounce ta, te, to ( た、て、と ) in the middle of a word as da, de, do ( だ、で、ど） (For example ： アナタ anata (you)→ アナダ anada , ヒ トリ hitori (one person)→ ヒ ドリ hidori , コ トバ kotoba (language)→ コ ドバ kodoba ).