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I work in an automobile the cars spares parts shoping center. Therefore I have to work when everybody else is having a day off. Saturdays and Sundays are hardly work days for me. However now I'm seeking а more interesting job, for example as a  marketing specialist in a the big foreign company., and Therefore I really need  to be able to speak English well. a good english. In the future I hope, to be a brand-manager in the FMCG company, like a P&G. Well, I think so. 
Please correct me.

12345

I work in the a cars spare parts shoping center. Therefore I have to work when everybody else have days off. Saturday and Sunday are hardly hard working days for me. However now I'm seeking а more interesting job, for example marketing specialist in the a big foreign company, and therefore I realy need a good english level. In the future I hope, to be a brand-manager in the fmcg company, like a P&G. Well, I think so
Please correct me.

In a cars spare part: in this case you have to use "a" because you´re not being specific about which center are you working in, if you give more information like the location of the cernter for example, then you will use "the" -> "I work in the cars spare parts shoping center located in ..."

Good job !