I want to work at a large company because large company can get more salaries than small company and 

they have a system for welfare of their employees.

I want to work in a large company because I can get higher salary than working in a small company. They also have employee welfare benefits. 

=> Corrected : The sentence is corrected grammatically.

-그는 페인트칠과 도배를 했다.

He painted and papered   →   He painted and hung wallpaper.​

-어머니 좀 바꿔 줄래요?

Could I speak to your mother? →  Would you put your mother on the phone.

=> Another way: Your sentence is okay and this is another way of saying it.

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-경기 침체로 우리 회사는 100명의 직원을 감축해야 한다.

Our company has to fire one hundred employees due to the economic downturn. 

 →​ We have to cut 100 workers because of the economic slowdown.  ​

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