German conversation is full of little words that add no facts, only tone: the Modalpartikeln. Das ist teuer states a price; Das ist ja teuer! is surprise — wow, that's expensive! Start with the three you'll hear most:

Particle	What it adds	Example
ja	surprise, or as we both know	Das ist ja teuer! — Wow, that's expensive! · Das weißt du ja. — You know that, after all.
doch	urging, or come on, surely	Komm doch mit! — Oh, come along! · Morgen ist doch Sonntag! — But tomorrow's Sunday!
mal	softens a request	Schau mal! — Have a look! · Kannst du mal helfen? — Could you give me a hand?

Particles never open a sentence. They sit in the middle, usually just after the verb and any short pronouns: Das weißt du ja. · Gib mir mal das Salz.