きのう　さいふ　なくした！　まじ　ありえない！

I lost my wallet yesterday! I can't believe it!

きのう　かのじょ　と　わかれた。

I broke up with my girlfriend yesterday.

まじ。

Really?

まじで。

Really?

If you joked which was supposed to be funny, but actually wasn't funny and nobody laughed, you can say,

すべった！

あした　テスト　か。べんきょう　しないと　やばい。

There will be a test tomorrow. I gotta study or screw up tomorrow.

When you eat something delicious, you can say,

これ　やばい！

References:

Basic Japanese Daily Phrases

https://anchor.fm/yuya-sato/episodes/Basic-Japanese-Daily-Phrases-emg8q3

Random Useful Expressions

https://anchor.fm/yuya-sato/episodes/Random-Useful-Expressions-en3j7l

Random Useful Expressions 2

https://anchor.fm/yuya-sato/episodes/Random-Useful-Expressions-2-enbbrk

Instagram:

https://www.instagram.com/yuyaman1986