〜かった

〜かった is the past affirmative for i-adjectives — "was X." 寒い → 寒かった = "[it] was cold." The pattern is: drop the final い from the adjective, then add かった.

This is the form every beginner forgets. The trap is that English-mind expects 寒いだった or 寒いでした — "was cold" + the copula "was." But i-adjectives carry their own tense morphology; they don't need a copula. The only role of です with i-adjectives is politeness — 寒かったです is the polite-register version of the same form.

The one exception: いい ("good") conjugates from its old form よい — past is よかった, not いかった. The negative よくない, past negative よくなかった — all from よ-, never い-. This irregularity touches most lessons because いい is one of the highest-frequency adjectives.

Past negative is 〜くなかった: 寒くなかった = "wasn't cold." Same drop-い, add-くなかった pattern.