If you are a beginner to Japanese, let’s first do a quick review of Japanese mora.
There are 46 basic characters and 100 different syllables. The consonants k, s, t, n, h, m, y, r, and w are used, along with the vowels a, i, u, e, o. See this wikipedia page for more information, we do not want to do a full discussion of Japanese phonology here. Suffice it to say that some of these are unvoiced consonants, consonants that do not require a sound to come from your throat.
An example of an unvoiced consonant is か, or “ka”. While the vowel does require a voice, the “k” does not.
However, you can specify using a “voiced consonant” by adding a “tenten”, or the two little dot diacritical marker. so, for example, か becomes が, or ga. So a tenten, which you will be taught tells you to turn “ka” into “ga”, for example, is actually to tell you to turn an unvoiced consonant into a voiced consonant.
This is important for rendaku, which we will address elsewhere, because some sounds morph into each other more smoothly than others. You will, in fact, discover that many of the things that make Japanese pronunciation seem odd and inscrutable do, in fact, only exist because the Japanese people don’t like to waste mouth energy. It is a fundamental concept in Japanese morphology, and shapes most of the concepts that you will learn by rote as you progress in Japanese.
So, for example, “ta” (た) turns into “da” (だ) because D is the voiced counterpart of T.
This is addressed in passing in most textbooks, and will not be treated as important, when, in fact, it is one of the most important concepts in learning basic Japanese. In fact, we would go so far as to say that this is the one thing that is the key to making most of the rest of the morphology make sense. See our pages on Handaku and Rendaku for further details.