The way in which the Dakuten (濁点) and Handakuten (半濁点) affect the pronunciation of kana must be understood if one hopes to master the kana. The Dakuten is quite simply two small strokes that appear at the top right of a kana which affect the way its pronounced. These two strokes (the Dakuten) mean that the kana needs to be pronounced ‘voiced’.
Dakuten exist for all the ‘k’, ‘s’, ‘t’ and ‘h’ kana, along with their Yōon equivalents. The way in which these kana are changed is as follows -
The ‘k’ sound becomes a ‘g’ sound (for example ‘ka’ (か) becomes ‘ga’ (が)
The ‘s’ sound becomes a ‘z’ sound (for example ‘sa’ (さ) becomes ‘za’ (ざ)
The ‘t’ sound becomes a ‘d’ sound (for example ‘ta’ (た) becomes ‘da’ (だ)
The ‘h’ sound becomes a ‘b’ sound (for example ‘ha’ (は) becomes ‘ba’ (ば)
There is also what’s known as the Handakuten, which modifies the ‘h’ kana into a ‘p’ sound. This is represented by a small circle that appears at the top right of the ‘h’ kana. The word ‘ha’ (は) for example would become ‘pa’ (ぱ).
There are some exceptions to the the way Dakuten characters are pronounced. These exceptions are with the dakuten for し (shi), ち (chi) and つ (tsu). し (shi) is pronounced じ (ji) in dakuten form, ち (chi) is also pronounced ぢ (ji) in dakuten form, and つ (tsu) is pronounced づ (zu) in dakuten form. The kana ふ (fu) is pronounced as if it took on its ‘hu’ form, and therefore simply becomes ぶ (bu).
Dakutens can also become Yōon characters, and the same exceptions also apply. For example the dakuten じ (ji) would become じゃ (ja), with the ‘i’ being dropped along with the ‘y’ (since the character originated as three letters). ぢ (ji) also follows the same rule.
Note: see Yōon Characters for further information.