せ
is the hiragana equivalent to セ
in katakana, so it stands for se. Both せ
and セ
were created from the same kanji, 世,
in fact. (せ
becomes ぜ,
i.e. ze, with the addition of two dots.)
Thus,すみません
reads sumimasen. There is no irregularity.
This is how to draw the character
し. It is composed of a single stroke.
The stroke is down, then curves up in a left-to-right diagonal.
し
is the
hiragana equivalent to シ
in katakana. It is pronounced shi. Whether spelled shi or si, the computer will
give you し.
(Its sound is neither [shi] nor [si] in English, of course.)
Given two dots, し
is voiced and becomes じ,
i.e. ji. Whether spelled ji or zi, the computer will given you じ.
(Its sound is neither [ji] nor [zi] in English, of course.)