Questions & Answers about Ta plotka o tobie to kłamstwo.
Why do we use tobie here instead of just ty?
The preposition o (when it means "about") requires the following word to be in the Locative case. The Locative form of the personal pronoun ty (you) is tobie.
Is this the same tobie we used before when saying things like "I am giving this to you"?
Yes! For the pronoun ty, the Locative form and the Dative form (used for "to you") happen to be exactly the same word: tobie. You know it is Locative here because it follows the preposition o.
Why does the sentence use to in the middle instead of jest?
When you are saying "Noun A is Noun B", you can use to followed by the Nominative (dictionary) form of the noun, which is kłamstwo. The alternative is to use jest, but then you would have to change the noun to the Instrumental case (jest kłamstwem).
Why do we have both Ta and to in the same sentence?
They look similar but do different jobs. Ta is a demonstrative pronoun meaning "this", and it is in the feminine form to match the noun plotka (rumor). The to in the middle is acting as a linking word that means "is".