Breakdown of Купи новую щётку, эта старая.
новый
new
старый
old
этот
this [f: эта, n: это]
купить
to buy (perfective)
Questions & Answers about Купи новую щётку, эта старая.
Why use the perfective купи instead of the imperfective покупай here?
In Russian, the perfective imperative (купи) is used to request a specific, single, completed action with a result. You want the person to get a new brush right now. The imperfective (покупай) would be used for a general rule (e.g., "always buy new brushes") or to urge someone to start an action they are hesitating to do.
Why is it новую щётку instead of новая щётка?
It is the direct object of the verb купи (buy), so it must be in the accusative case. For feminine singular words, the nominative adjective ending -ая changes to -ую (новую), and the noun ending -а changes to -у (щётку).
The English translation says "this one," but the Russian just says эта. Where is the word for "one"?
Russian does not have a direct equivalent to the English placeholder "one." Instead, it simply uses the demonstrative pronoun or adjective on its own. Because щётка is feminine, using the feminine эта (this) and старая (old) is perfectly enough for the listener to understand you mean "this brush is old."
If I wanted to say "Buy mom's new brush," how would I use the possessive suffix -ин here?