"I really don't know."
Translation:De verdad no lo sé.
"Realmente no sé" rejected and reported 08/03/2018. I reported this because the correct answer given was "Realmente no lo sé" but the origin english sentence did not state or imply a masculine DOP (him, it, you singular formal) or neuter DOP (idea or concept) for what was not known. The english sentence simply states that the subject (I) is in the present (sé) state of not knowing. Nothing more! Simplified if the sentence "I know." = "Lo sé." = "I know it." then "I swim." = "Lo nado."= "I swim it." I think not.
Both of the above articles concur that the use of and removal of ambiguity when using DOPs require context but that was not given or required in the origin english sentence. The DOP was introduced in the Spanish translation to change the meaning of the sentence even though there is already a valid Spanish construct that provides the correct semantics for the given English sentence. "Realmente no sé"