What is "Resumptive Lo?"In Spanish we often see sentences with a mysterious lo in them. For example consider this dialogue:
¿Estás listo? (Are you ready?)
Lo estoy. (I am.)
It's fair to ask "what's the lo doing there?" It seems to mean "I am it." This lo is what linguists call an expletive, and it cannot be deleted.
ExpletivesSometimes the syntax of a language forces you to put a word into a sentence even when the semantics don't require it. When that happens, you use an expletive, which is just a dummy word that doesn't mean anything.
In English, you find expletives in expressions like "It is raining," "There is trouble," and "Do you see?" The "It," "there," and "do" are there because English syntax requires something in those slots. The expletive disappears whenever there is a valid word for that slot. "Have you seen?" is valid but *"Have you done seen" is substandard.
One of the challenges of learning Spanish is learning to leave the English expletives out. "It is raining" just becomes Llueve. "There is trouble" becomes Hay problemas, etc. It shouldn't be a big surprise, then, that another challenge is learning to put the Spanish expletives in.
Three places that need lo
For words like ser, estar, and parecer, Spanish doesn't allow you to leave out the complement. That is, you can't just say "I am" or "he is"; you have to say what you are or what he is. Even though lo doesn't mean anything, it satisfies the syntax. If you just say *Estoy, then you've made an error on a par with saying "Is raining."
Transitive verbs have the same problem. You cannot omit the direct object.
¿Quién tiene una carta? (Who has a letter?)
Lo tengo. (I have).
This doesn't mean "I have it," obviously, since carta is feminine, and "I have it" wouldn't make sense in this context anyway. ("I have one" would also translate this correctly.) Again, *Tengo all by itself is wrong.
Finally, when haber means "there is" you have to say what there is.
¿Hay algo que comer? (Is there anything to eat?)
Lo hay. (There is.)
Likewise, *Hay, by itself is an error.
When do you use it?That answers why you have to use it, but it doesn't explain when you would want to. You use the resumptive lo when the missing element simply repeats something earlier in the sentence or even in a previous sentence. In all my examples above, the lo represents something the first speaker said and which the second speaker doesn't need to repeat because "it's obvious." Nothing stops you from repeating what was said, of course, but almost no one does that--in any language.
This use of lo to "reuse" something from earlier is why this is called "resumptive lo."
You can use resumptive lo inside a single sentence. This example is from Duolingo:
Este plato es bonito, pero ese no lo es. (This plate is pretty, but that one is not.)
It's very tempting to say ese no es, because that's what we do in English, but it's wrong. A form of ser has to have a complement--even if it's just lo.
For more details, read "A New Reference Grammar of Modern Spanish: Fifth Edition" (Butt and Benjamin, 2011, section 7.4 "Lo as a neuter pronoun").