Learn Russian: Я покупаю мало масла, а он покупает много сыра. - I buy little butter, and he buys a lot of cheese.

This is because of the words мало (little) and много (a lot). In Russian, these quantity words require the noun that follows them to be in the Genitive case. For masculine words like сыр and neuter words like масло, the Genitive singular ending is -а.

Will много and мало always take the Genitive singular form of a noun?

Not always! You use the Genitive singular for uncountable nouns, which are things you measure rather than count individually, like масло (butter) or сыр (cheese). If you were talking about countable things, like many dogs or few houses, you would use the Genitive plural instead.

The English translation says 'and he buys', so why does the Russian sentence use а instead of и?

The word а is a contrasting conjunction. It points out a difference or comparison between two things. Because we are contrasting what 'I' buy versus what 'he' buys, а is the natural choice. It acts a bit like 'whereas' or 'but' in English.

How does the verb покупать (to buy) change its endings in this sentence?

The verb changes its ending depending on who is doing the action. For я (I), the ending is -ю, giving us покупаю. For он (he), the ending is -ет, giving us покупает.