Be careful here. Remember to distinguish between the demonstrative adjective "ten/ta/to/ty" etc. on the one hand, and the demonstrative pronoun "to" on the other.
It is the demonstrative adjective that is translated as either "the" or "that" rather than "this". This DL course is quite consistent on that point.
In contrast to the adjective, the pronoun "to" is invariable and is used for both singular and plural -- that is, for both "that is" and "those are". It can also be translated as "it is" (singular) or "they are" (plural).
But DL sometimes also translates the pronoun (not the adjective) "to" as "this" (singular) or "these" (plural). Perhaps this reflects a difference between English and Czech in how the pronoun is used.
For example, when introducing your sister standing next to you, in English you would always say "This is my sister", not "that". But perhaps in Czech you would say "To je moje sestra"?
Or would it be better to say "toto" or "tohle" when introducing her, and reserve "to" for pointing her out across the room?