Learn Russian: Он постирал оба рукава.

The word for 'both' changes depending on the gender of the noun. Use оба for masculine and neuter nouns, and обе for feminine nouns. Since рукав (sleeve) is a masculine noun, we use оба.

Why is рукава in the genitive singular instead of plural? Doesn't 'both' imply more than one?

In Russian, the word оба behaves exactly like the number two (два). When used in the nominative case (or the accusative case for inanimate objects, as seen here), it requires the following noun to be in the genitive singular. Therefore, рукав becomes рукава.

Does the pronunciation of рукав change when it becomes рукава?

Yes, the stress shifts. The nominative singular is рукАв (stress on the 'а' in the middle), but in the genitive singular form рукавА, the stress moves to the final 'a'.

What case is the whole phrase оба рукава in?

The phrase as a whole is in the accusative case because it is the direct object receiving the action of washing. Because sleeves are inanimate objects, the accusative form of оба looks exactly like its nominative form. If the object were animate (like 'both brothers'), the accusative form would change to look like the genitive plural (обоих братьев).

Why is the verb (perfective) used instead of (imperfective)?