
The 4th set of Hiragana is the Monographs (gojūon) combinations with t. Each vowel (a - i - e- o - u) gets paired with the consonant to form a new syllable. The exceptions here are ti (which is pronounced and written in Romaji as chi) and tu (which is pronounced and written in the Romaji as tsu)
Pronouncing ta - chi - tsu - te -to