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I was speaking to a Spanish native on Speaky and she used the following sentences
Es loco lo sé. It's crazy I know.
Eso lo supe en la universidad. I knew that at University.
This use is confusing to me because if I was translating these English sentences to Spanish I would have omitted lo in both cases.
Different things are happening there. Your first example is actually not correct in Spanish as written, as it's not, I think, in English; there should be a comma in there, because there are two propositions:
Es loco, lo sé. = "It's crazy, I know."
The Spanish equivalent of "(...), I know" is not the literal translation. Lo sé properly means "I know it", where "it" refers to the fact that you've stated just before that. You can also add ya and either leave the pronoun or drop it (ya lo sé or ya sé = "I already know"). You cannot under any circumstances say just sé "I know" in a phrase like that.
On to the second sentence:
Eso lo supe en la Universidad. = "That I knew at University."
There you have a different case. Following the usual word order in Spanish (subject - verb - object) you would say (Yo) supe eso en la Universidad. Note there's no pronoun lo there. But that's not what you're saying here. You're moving the direct object (eso) to the front in order to make it the topic of the sentence. When that happens in a sentence like this, it's as if you have left the direct object slot empty, and since it has to be filled, we use the pronoun (in this case, lo). In way what you're saying is equivalent to
En cuanto a eso, lo supe en la Universidad. = "As for that, I knew it at University."
Some examples of this latter kind:
The pronoun is compulsory in all these cases. In English you can say things like "The table they will send tomorrow" or "Your problem I'll attend to later", but not in Spanish. Note also how in English these ideas would be more commonly conveyed using the passive voice ("The tree was felled yesterday"), which in fact achieves the same thing (turning the object into a subject and moving it to the front, which makes it the topic). The passive voice is not nearly as common in Spanish, least of all spoken Spanish.
In online communication, people often omit punctuation. The first sentence is actually
Es loco, lo sé. It's crazy, I know. OR It's crazy; I realize that.
For the second sentence, I'll give an alternate, freer translation that will hopefully help to convey the idea:
Eso lo supe en la universidad. I came to know that at University.
You would like to know why "lo" is being included, since when you tried to go from the English idea to a Spanish sentence, you didn't include "lo."
The answer is that it is idiomatic to include "lo" in sentences such as these.
Note that object pronouns typically go before the verb in Spanish, for example
Yo tengo tu libro --> Yo lo tengo.
Yo sé que tú tienes mi libro --> Yo sé que tú lo tienes.
When "it" or "that" are used as indicatives of the subject they are replaced by Lo/La .
And interesting case is the "le" that is used when the indication refeers to someone else.
You can research more about this with the Direct object and Indirect object, and to understand this i would recommend to read in Spanish rather to trying to translate from english.
Also for this specific case maybe "Dora" is a good example , with her signature Phrase "We did it/lo Hicimos"
