Questions & Answers about Каждый месяц я выбираю новую косметику.
Why do we use the accusative case for каждый месяц even though there is no preposition?
To say you do something "every [time period]" (like every day, week, or month), Russian uses каждый and the time word in the bare Accusative case, meaning no preposition is used. Because месяц is a masculine inanimate noun, its Accusative form looks exactly like its dictionary form.
"Cosmetics" is plural in English. Is косметику plural in Russian?
No, косметика is a singular feminine noun in Russian. It works like a mass noun (similar to "water" or "makeup" in English). Because it is singular, the adjective describing it must also be singular, which is why we see новую instead of the plural новые.
Why does the phrase end in -ую and -у (новую косметику)?
The phrase "new cosmetics" is the direct object of the verb выбираю (it is the thing being chosen), so it needs to be in the Accusative case. The feminine dictionary form новая косметика changes its endings to -ую and -у in the Accusative.
How does the phrase каждый месяц affect the choice of verb here?
Expressions of frequency like "every month" show that the action is a repeated habit. Because the action happens regularly, we have to use the imperfective verb выбираю (I choose) instead of a perfective verb, which would only be used for a one-time completed action.