Друг друга: What Happened to the Friend?

Alright, I hope you got my joke. The phrase друг друга may look connected to друг, meaning "friend," but it's a fixed expression — a reciprocal pronoun — meaning "each other":

• Мы лю́бим друг дру́га. — We love each other.
• Они́ помога́ют друг дру́гу. — They help each other.

Like many Russian words, this little phrase changes according to case. But the good news is: only the second part changes. It has no nominative form, because друг дру́га is never used as the subject of a sentence. Let's look at the forms and examples in a table:

As you can see, the second word, друг, has very predictable and regular declension forms. In the prepositional case, you can also see how prepositions work inside this phrase. Here's a table with other prepositions you may use:

How do you know which form to choose? Look at the verb or the preposition. The phrase друг дру́га follows the same logic as other Russian words: the verb or preposition asks for a certain case. If Russian cases still feel a little mysterious, I explain the whole system step by step in my guide: Russian Cases: The Logic Behind the System.

So yes, друг дру́га looks a little funny at first. But the logic is friendly: the first друг stays the same, and the second part simply follows the case. Now go to the Exercises tab at the top of the page and practise it a little — your future Russian-speaking self will thank you :)