"I like it."
Grammar. In different places in sentence after different verbs or prepositions, "to" takes different cases. Tego is genitive, to is nominative=accusative.
Lubię needs accusative- Lubię to Nie lubię needs genitive - nie lubię tego.
Accusative changes to genitive when the verb is negated.
If you said "Lubię tego", it implies "tego" is masculine animate (since "lubię" requires accusative, "tego" looks like genitive, and only in masculine animate genitive=accusative). It would then mean "Out of these guys, I like this one", or "Out of these grammatically-masculine animals, I like this one"
Yes it is. Frankly, it's even better in most contexts that are not 'liking sth on facebook'.
Shouldnt it be "Lubię go/je/ją"?
When we say "I like it", "it" refers to the 3rd person singular pronom, right?
If we use "to" (Acc) / "tego" (Gen), then we are using the demonstrative pronom ten-to-ta, no?
I don't get how to differenciate them in the exercises :/
Yes, those seem correct, added. The problem is the lack of context here and not knowing what 'it' is.
"to" in this sentence is rather the dummy pronoun that refers to... to the situation, maybe? Frankly, apart from the fact that "Lubię to!" is the most famous button on Facebook, I don't know where else it could be used.