"Kateřina sent me a long letter."

Translation: Kateřina mi poslala dlouhý dopis.

[JBHayven]

What about "Kateřina poslala mi dlouhý dopis"?

[endless_sleeper]

Mi has to be the second word.

[artem10436]

Muže být "kateřina poslala mně dlouhý dopis

[VladaFu]

No, it sounds strange. Although mně is not a strict clitic it still comes either in the second position or in the strong (first or final) positions.

[stan602994]

I think yes. It should be accepted. I reported on that

[VladaFu]

Please check for typos before any reporting ("kateřia poslala mně dolhý dopis")