It is でした, Should be in you textbook. I highly recommend reading Tae Kim's Japanese guide to Japanese language on his site. Note that です is used not only grammatically but also for politeness, like, with "true" adjectives. Then it won't work that way because an adjective has a past form of its own: the past form of 「いい」 is　「よかったです」, not　「よいでした」.

Me gustaría recomendarte una página en que un español enseña japonés en español. Es "Kira Teaching", echa un vistazo si tienes interés.

ありがとうございます!! I look in some web dictionaries, but I didn't find the answer...that's because I asked here. and thank you for the extra information, I'll keep it in mind!!

It is でした, which, I am pretty sure, should be in any textbook. Did you really mean that? Note that since 「です」 is used also for politeness, it may not work 100% (for example, with "true" adjectives which have a past form of their own).