By the way, another, hopefully more easily digestable explanation for the tsu vs. tu thing is the following:
The natural arrangement of the Japanese alphabet is in table form, where the rows of the table indicate the "first letter" (consonant) if you were spelling the letters in English, and the columns represent the "second letter" (vowel) if you were spelling the letters in English.
The rows of the table are [none], k, s, t, m, h, n, y, r, w.
The columns of the table are a, i, u, e, o.
With only a couple exceptions, every single combination of these letters is a valid Japanese "letter". For example, k+u = ku = く. n+e = ne = ね. etc.
The reason tsu/tu (e.g. つ
is frequently written as "tu" by native speakers is because it creates more consistent rules for transliteration. ta, ti, te, and to do not have an "s" sound in the middle so it would actually be blatantly wrong to write those as tsa, tsi, tse, or tso. Those
sounds don't even exist.
So simply for the sake of consistency in spelling, "tu" is written instead of "tsu".
Granted the linguistic explanation is more forml and more "correct", but hopefully this explains it in a way the layman can understand.