This is the 3355th most frequent Japanese word.
暑かっ
"暑かっ" is the stem form of "暑かった," meaning "was hot" (referring to temperature).
暑かった is used as the past tense of the adjective '熱い', describing the weather condition in a personal statement about the previous day.
昨日は本当に暑かった。
Yesterday was really hot.
暑かったです is the polite past tense form, expressing respect while stating the heat in the room at a specific time.
その部屋は私が入った時とても暑かったです。
That room was very hot when I entered.
暑かった is used within a reflective statement, indicating a repeated condition during past summers through memory.