Hi. This is a very good question. Please let me try to answer. As you know, hiragana and katakana is a table based on 5 vowels, a, i u, e, o. For S sound, in theory, we should have: sa si su se so But Japanese has no sound for si and hence it's changed to "shi" which is considered to be the closest sound to "si" in Japanese. Technically "shi" is correct but people can use "si" for Romaji also since it's shorter. But both should be pronounced as "shi". Same goes for T sound. ta ti tu te to But no equivalent sound in Japannese for "ti" and "tu" so we substitute them with ta chi tsu te to but again, it's ok to write "ti" and "tu" to mean "chi" and "tsu" as it's shorter and easier. As you can see now, "shi" and "chi" are totally differently pronounced (and therefore uses the different hiragana/katakana characters. (し　ｖｓ　ち）　（シ　ｖｓ　チ） Similarly, Z sound, za zi zu ze zo No sound for "zi" so "ji" is substituted. Lastly, D sound, da di du de do No sound for "di" so "ji" is again substituted as it is considered the closest sound to "di". I hope this helps.

Hanoun seems right. Also don't ever say "Jap," in English. It's considered to be a racial slurr. "Chi" is pronounced like "che" in the English word, "cheek." "Shi," is pronounced like the English word, "she." "Zi," and "di," don't exist in modern Japanese. "じ" and "ぢ" are pronounced in the same way. "ぢ" is less common. In writing in kana, you have to remember whether to use "じ" or "ぢ." The same exists with "ず" and "づ." They are pronounced the same way, "zu." "づ," is less common and you still have to remember which one to use.

hi chahrazed take it easy it is very simpl there r not ti in hiragana letters so we found 'chi' pronounce 'tchi' in frensh 'shi' it's the same , for 'ji' = 'dji' ' 'di' it's same good luck and keep on!!