"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" (transitive) but the origin English sentence is (intransitive). 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.
My dictionaries concur that both "saber" and "to know" can be used transitively and intransitively.
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"
The "lo" appears to be used because you are really saying: "I really don't know it". So the lo refers to "it".
Is it implied? Maybe not. "I really don't know" and "I really don't know it" are not the same in meaning.
I know we are supposed to be learning things conceptually and not "word for word" but it bugs me as I build my vocab sheets when the actual meaning of the word is obscured - "verdad" doesn't mean "really" it means "truth". So, yes, this phrase is useful for when you want to convey that you know or don't actually know the facts or the truth about something, and can ideologically translate to English as "I really don't know", but this phrase actually means "I don't know the truth" and that is still conceptually compatible, so why not just use it, or include it in the definition?
you're confused. this phrase doesn't mean i don't know the truth it means "in truth, i don't know (it)"
lo is used with se' when there is no reference to what you know or don't know (like an "it"), as in this sentence. yo se' or just se' would be used if you know what the "it" is, such as a certain thing or person. For example, if this sentence said "I don't know this word", lo would not be used because you know what the "it" is, rather it would be "yo no se' esta palabra" or just "no se' esta palabra".