On the contrary恰恰相反
The book is not easy. On the contrary, it is rather difficult.
It wasn't a good thing; on the contrary it was a huge mistake.

Once一旦
Once a beginning is made, the work is half done.
Once you begin, you must continue.

Otherwise否则
Do what you have been told; otherwise you will be punished.
You'd better go now, otherwise you'll miss the train.

Provided 如果…
I will come provided I have time.
Provided it's fine we will have a pleasant holiday.
Scarcely…when(hardly…when)一…就（句子经常要倒装）
Scarcely had he gone out when it began to rain.
I had hardly arrived the platform when the train left

When
1) 当，当…之后
I will be there waiting for you when you arrive.
I started my elementary school when I was 6.
2) 就在那个是时候 (常表示两个动作几乎同时发生或者when引导的动作在主句的动作发生之时)
I was leaving the house when someone knocked at my door.
It was on my way home when I met my classmate.
3) 既然
I can’t tell you anything when you won’t listen.
We need to adjust the prices of our products when prices of raw materials rise so sharply.
4) 虽然
I lose temper when I shouldn’t.

While
当…时期
Please don’t talk so loud while people are reading.
1) 而
You clean the window while I sweep the floor.
2) 虽然
While I like the color of it, I don’t like this shape.