How to say "I forgot" in Japanese わすれてた VS わすれた

 "I forgot" in Japanese

 I forgot about it! (It slipped my mind.)

You can use it when you had forgotten about it and you just remembered it.

Casually

忘わすれて(い)た。

(Wasurete(i)ta)

 Formally

I completely forgot. 

Casually ①

完全かんぜんに忘わすれて(い)た。

(Kanzen ni wasurete(i)ta)

 Formally①

完全かんぜんに忘わすれて(い)ました。

(Kanzen ni wasurete(i)mashita)

Casually ②

うっかり忘わすれて(い)た。

(Ukkari wasurete(i)ta)

Formally②

うっかり忘わすれて(い)ました。

(Ukkari wasurete(i)mashita)

What about 忘わすれた? 

you can use 忘わすれた when...

① You forgot to bring something. 

Ex) かさ(を) 忘わすれた!

(Kasa o wasureta)

- I forgot my umbrella.

② You forgot something and still can't remember it.

Ex)本ほんの名前なまえ(を)  忘わすれた! 

(Hon no namae o wasureta)

- I forgot the name of the book.

③ You left something somewhere.

Ex)電車でんしゃに かばん(を)  忘わすれた! 

(Densha ni kabani o wasureta)

- I left my bag on the train.

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