Originaltext - Latein - istud ipsu

"istud" originou a palavra "isto"
e
"ipsu" originou a palavra "isso"
em português e gostaria de saber a diferença entre estas duas.
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"istud" is the root of the word "isto"
e
"ipsu" is the root of the word "isso"
in portuguese and I would like to understand better the diference beetwen this two words.