That reminds me
そう言えば That reminds me, everyone please use 6 idioms in your written speech.
That'll be the day
まさか、そんなことはあり得ない！、それはあり得ないことだ！(That will never happen). Futenma Air Station moving out of Okinawa?? That'll be the day!
Track down
突き止める To look for and find someone. You can track down people using the Internet these days.
Turn out
(結果として)...になる、となる Wow, many of you have turned out to be beautiful young ladies!
Watch your mouth
言葉に気をつけろ！(Be careful of your language.) Hey, watch your mouth, Ms. S!
The way I see it
私の考えでは，私の意見では... (I think, In my opinion...) The way I see it, if you put your mind to it, you can become successful.
What if
したらどうする？、なったらどうする？ What if I don't practice my English? Well, if you don't practice, you'll lose your ability. Please practice often!
What on earth
いったい何?! What on earth are you talking about? (We also say, "Why on earth?)
Why bother
無理だからあきらめたら？、成功する可能性があんまりないから、あきらめたら？ (No point in trying because you can't succeed) Why bother asking her out on a date? She'll just say no.