The letters t and d, and s and z represent alveolar consonants. The former are plosives and the latter are fricatives, both respectively voiceless and voiced.
The sounds of these letters are:
t: table, teacher, trainers;
d: digital, dog, drive;
s: casual, spring, student;
z: size, zebra, zero.
T and d or s and z are alveolar sounds, to pronounce them you need to touch the alveolar ridge (the back part of what holds your teeth) with your mouth, creating a block or a small opening with the teeth.
The sound of t and d is produced when the tongue touches the upper teeth, creating then a gap by which air passes. These are called plosives, as the block obstruct slightly the passing of airflow.
- t is voiceless so the vocal cords do not vibrate during the articulation of the consonant;
- d is voiced so the vocal cords vibrate in the articulation of the consonant.
The sound of s and z is produced when the tongue touches the lower teeth, also creating a gap by which air passes. These are called fricatives because the mouth is almost closed, but not completely, so the air can still flow.
- s is voiceless, so the vocal cords do not vibrate during the articulation of the consonant;
- z is voiced, so the vocal cords vibrate during the articulation of the consonant.
External link to Letters: T-D, S-Z exercises (1157).