Объясните , когда пишется some/any/an в предложениях(поподробнее

Пишем:
Some - с неодушивленными предметами (в утверждении и вопросе)
Any - c неисчисляемыми. (Только в вопросе и в отрицании)
an - перед неопределенным словом, начинающимся на гласный звук. Пример:
an elephant.

Some - употребляется в утвердительных предложениях перед исчисляемыми существительными во множественном числе и перед не исчисляемыми существительными.
Any - обычно употребляется в вопросительных и отрицательных предложениях. 
Неопределенный артикль a употребляется с исчисляемыми существительными в единственном числе

I go to school five days a week, so I have two days off — Saturday and Sunday (I’m lucky, because some other pupils have the only one day off). During the week I am very busy, so I like to have a rest on weekend. I am not an early riser and it is a rare Saturday or Sunday when I get up before 9 o’clock. I enjoy staying in bed, when I don’t have to hurry anywhere. We have late breakfast at 10 and watch TV. Usually we have something tasty: meat salad, fried potatoes, chicken, cake or pie. If the weather is fine, I usually do not stay indoors, I and my dog go outside. Often we go to the park and play there. If the weather is rainy and gloomy, I stay at home and watch TV, listen to the music, read the books. After dinner we go visit our grandparents or relatives, or just simply take a nap. Sometimes when my friends call me we go roller — blading near the Opera theatre. I like roller — blading very much, I think it is a lot of fun. In the evenings I like to watch video and music programs. There is a big armchair in my room right beside the lamp with blue shade. If it is cold I like to sit there with cup of coffee and read. Sometimes I do something special on weekends: go to an art exhibition, to the theatre, to the concert. I always go to bed late on Sundays, and Monday morning is the nastiest thing through all the week. I like weekends very much, because I can rest and gain some energy for the next week. Джерело: http://dovidka.biz.ua/miy-vihidniy-den-na-angliyskiy-movi/ Довідник цікавих фактів та корисних знань © dovidka.biz.ua

The economy — is the science, studying regularities of our life.
Economists study the system in which we live and the conditions that influence on it.

They describe the economic events and deduce the economic laws and formulas.
The science called «the Economy» is connected with the material needs and requirements of the humanity.
The subject “Economy” is not separated from the other disciplines.
On the contrary, it is closely connected with its.
So, you cannot learn the Economy without studying Economy statistics, Mathematics, Marketing, and History.
Besides the science “Economy” is enough comprehensive and includes some other disciplines: Finance and Credit, Accounting, Trading business, Economic management and so on.
The economic professions vary according the world progress.
For example, a specialist in the sphere of economy should also be well qualified user of the computer and know the foreign languages.