to arrange things in the wrong order

I've arranged the books alphabetically so don't muddle them up. 我已经把书按字母顺序整理过了，不要弄乱了。

to think that a person or thing is someone or something else because the two people or things are very similar

I often muddle up Richard with his brother. 我常常分不清理查德德和他哥哥。

It's easy to muddle up some Spanish and Italian words. 很容易将一些西班牙语词汇和意大利语词汇混淆。