Use katta and datta to make adjectives plain and past. Datta is the universal plain form of deshita, and can be used at the end of many sentences to make them plain and past. Katta is for true adjectives only, however, and is added after removing the final i.
Here are a few true adjective examples:
- Kyou wa atsukatta! (It was hot today!)
- Suugaku no shiken wa totemo muzukashikatta. (The math test was very difficult.)
- Kinou no ryokou wa tanoshikatta. (Yesterday's trip was fun.)
And here are some quasi examples:
- Kinou byouki datta. (I was sick yesterday.)
- Juu nen mae ni John wa binbou datta. (Ten years ago John was poor.)
- Rekishi no shukudai wa kantan datta. (The history homework was easy.)
Now, having done this, you can further conjugate using the endings and combinations applicable to other plain forms, like those in Lesson 2:
- Samukatta deshou? (It was cold, wasn't it?)
- Chiisakatta hazu. (It was supposed to be small.)
- Kare wa totemo ganko datta rashii. (It seems he was very stubborn.)
If you are ending a sentence with an adjective and want to make it past and polite, just add desu after katta in true adjectives, and use deshita instead of datta with quasis:
- Kaigi wa nagakatta desu. (The meeting was long.)
- Shokuji wa kanzen deshita. (The meal was perfect.)
Note: The adjective ii (good) is not conjugated into the past tense. Use yokatta to say that something "was good."
Finally, in case you need the plain past negative, just change the nai covered in Lesson 2 to nakatta :
- Kinou wa atsuku nakatta. (It wasn't hot yesterday.)
- Sore wa kantan dewa nakatta. Totemo muzukashikatta! (That wasn't easy. It was very difficult!)
To upgrade these to polite, use arimasen deshita instead of nakatta.