““Group 1 Verbs （五段ごだん活用かつよう）”

Group 1 verbs undergo a sound shift in the final syllable before attaching different endings.

For example: 話はなす becomes 話さない (negative), 話します (polite), 話そう (“let’s speak”), 話せば (“if can speak”), and 話せる (“can speak”).

These changes are seen daily — especially when talking about ability, politeness, or making suggestions.