"This pen is the best."

[shmeur]

Isn't 'tiu' 'that'? Because in other sentences, I would put 'this' for 'tiu', and I would be marked incorrect.

[mrfinnsmith]

Yeah, that was my understanding as well.

[VincentOostelbos]

Really? I was marked as correct. But I suggested for some sentences that it should perhaps be marked as incorrect, as in Esperanto "ĉi" is used to change "that" into "this" (and similar for the other correlatives). However, perhaps it's not quite that strict in Esperanto, as rapn21's comment suggests.

[rapn21]

It can be either this or that depending on context. You should have been marked correct.

[ChadScherr]

It would be helpful to have some more detailed section notes about when to use 'tio' vs 'tiu' vs 'tiu ĉi'