"The letters in this newspaper are too small."

Translation: Huruf di koran ini terlalu kecil.

[AnitaRRC]

Why is huruf-huruf wrong here? Thought that is how you do plurals?

[Stewart737090]

sama sama!

[PaulGilbert731]

I am also confused about that rule. When is it acceptable to use the double word formation of plurals? Its use seems a bit arbitrary

[Eric50436]

It seems to me that you use double-word plurals if you are talking about a specific set of plural things, often owned by someone or something. If you are talking about a thing in general, you can keep it at single word. Contrast:
My bunnies eat grass = kelinci-kelinci saya makan rumput
Vs.
Bunnies eat grass = kelinci makan rumput

In this case, you pretty much know that a newspaper's content is made up of more than one letter, so we are generically talking about the letters in the newspaper.