"No sé lo que es": meaning and grammar

Question

"No sé lo que es eso" means "I don't know what that is."

But does "No sé lo que es" mean "I don't know what it is." or is it even grammatically correct?

Answer 1

Both options are correct, but the first one sounds more natural, maybe because you need some context to see how the second could work. Consider (i):

A: ¿Quieres una ensaimada? ^{(Do you want an ensaïmada?)}
B: No sé lo que es eso. / No sé lo que es.

Here, "no sé lo que es" sounds like an incomplete thought in some dialects. In others, it sounds okay. Now consider (ii):

A: ¿Qué quieres? ^{(What do you want?)}
B: Te pediría una ensaimada, pero no sé lo que es. ^{(I'd order an ensaïmada, but I don't know what it is.)}

Here, "no sé lo que es" sounds okay because of the proximity of the object ("ensaimada"). Note that "no sé lo que es eso" also works, but it sounds a little bit too wordy for my liking.

In conclusion, as a stand-alone sentence you can use both (see i). You can also use "no sé lo que es" when the object of "es" is somewhere over there, especially as a part of the same sentence, but you wouldn't normally use "no sé lo que es eso" in that case because it would sound repetitive, albeit perfectly grammatical (see ii). You can also omit that "lo" in "no sé lo que es eso", but then the function of "que" changes from a conjunction to an interrogative particle which means you have to add a stress mark:

No sé qué es eso.

The meaning is the same as "no sé lo que es eso", and it works the same way too. You can also omit the "lo" in your second sentence and just say "no sé qué es", but that's not that common and generally involves a change in the verb tense emphasizing the incertitude ("no sé qué será"). For more on "omitted lo" see the examples below.

Finally, the difference between both sentences is subtle, and if you add all the dialectal variations to that the whole thing gets really hard to explain. Most of the times it's about how both sentences "sound", so that one sounds okay and the other sounds off (and again, how both "sound" also depends on the country you are visiting or the dialect you are learning).

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As a side note, keep in mind this only works when both are complete sentences. For example, the "it" in "I don't know what it is" can work as a dummy pronoun in sentences like:

I don't know what it is like to be rich. = No sé cómo es/cómo se siente ser rico.
I don't know what it is that you don't like. = No sé qué es lo que no te gusta.

In such cases the substitution isn't that simple, and it may not always work. You need a case-by-case analysis to know. Also, none of the above admit "lo" before "qué".

There are also a lot of expressions with "I don't know what it is with...". The Spanish equivalent expressions are varied, and all of them admit a "lo" before "qué" (but remember that if you add "lo" you have to delete the stress mark):

I don't know what it is with this city in the winter, but I love it = No sé qué tendrá esta ciudad en el invierno, pero me encanta.

I don't know what it is with females, but I'm not too good at that shit (Kanye West's song) = No sé qué me pasa con las mujeres/No sé qué tienen las mujeres, pero no se me dan nada bien.

I don't know what it is with these people. They are all nuts. = No sé qué le pasa a esta gente. Están todos locos.

I don't know what it is with you and that book, but you never seem to put it down = No sé qué te dio con ese libro, pero nunca pareces dejar de leer.