We had to stop working because of the heavy snow.

->Because the heavy snow,we had to stop working

If you don't start working hard now , you won't be able to pass the final test.

->Unless start working hard now , you won't be able to pass the final test.

I'll be on vacation next week, so I won't attend the meeting.

->Since I'll be on vacation next week,I won't attend the meeting.

The water bill was enormous because there was a crack in the water pipes.

->Because of the crack in the water pipes, the water bill was enormous

He is a fluent speaker of English.

-> He speaks English very fluently.

She hasn't gone out with him for almost two years.

->They stopped going out with together for almost two years.

Nam won't be successful unless he works hard.

->If he doesn't work hard Nam won't be successful

Writing:
We had to stop working because of the heavy snow.
->Because.... the snow is heavy, we had to stop working ( stop ving: dừng làm gì ) 
If you don't start working hard now , you won't be able to pass the final test.
->Unless .....you start working hard now, you won't be able to pass the final test ( unless= if not )
I'll be on vacation next week, so I won't attend the meeting.
->Since (ở đây hiểu là bởi vì ).....i'll be on vacation next week, i won't attend the meeting
The water bill was enormous because there was a crack in the water pipes.
->Because of... the crack in the water pipes, the water bill was enormous ( to lớn/ khổng lồ ) ( because of +N= because +s +v +o )
He is a fluent speaker of English.
->He speaks english fluently ( trôi chảy )
She hasn't gone out with him for almost two years.
->They stopped going with him for almost 2 years
Nam won't be successful unless he works hard.
->If nam doesn't work hard, he won't be successful