Personal pronouns (já, ty, on, ona, my, vy, oni) are usually omitted because the Verb Conjugation already indicates the person. It sounds very unnatural to use these pronouns in everyday conversation.

ty Snídáš doma nebo v práci?

já Pracuju tady ve firmě.

They are used only when there is a need for emphasis (usually when there are two different people in the sentence).

Já snídám doma, ale Alena ne.

Ona pracuje v bance a já v kině.

On the other hand, the demonstrative pronoun to is never omitted. It is usually used in “To je…” and “To není…” sentences.

To je velmi dobrý nápad!

To není moje taška.