Lesson 3: Diacritics
Japanese has two diacritics, which change the sound of the hiragana characters they mark, to form new hiragana characters.
The dakuten is the voicing mark, which turns the unvoiced sound it marks into a voiced sound. Sounds that are unvoiced, such as t, are pronounced without vibration of the vocal cords, whereas sounds that are voiced, such as d are pronounced with vibration of the vocal cords.
For example, the hiragana character for the sound ta is た. Given that we know this, we can also know how to write the character for da, because the only thing we need to do is add a dakuten to the character for ta, such that the character for da is だ.
The handakuten is the half-voicing, used for changing syllables that start with an h into syllables that start with a p. For example, ha is は and pa is ぱ.