"wasureta" and "wasurechatta"
I heard that people in Japan use both of this words in similar sentenses...but I don't know, is there big difference between them?

Question

Answer

"wasureta" means only "I forgot it". "wasurechatta" is a short version of "wasurete shimatta".
This means a stronger expression like "Gosh, I really forgot it. Am I stupid." or the like.

"..chatta/...te shimatta" are very useful expressions to say something happened without
or against my intentions.

Answer

... pass...    who is the downstairs can answer it!

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