dass

dass means that, as in "I think that German is interesting". It belongs to the same family as weil: it opens a subordinate clause, so the conjugated verb goes to the end.

Main clause · comma · dass · subject · … · conjugated verb (end). Exactly the weil pattern.

• Ich glaube, dass Deutsch interessant ist.

  I think that German is interesting.

  ist at the end

• Ich weiß, dass er heute arbeitet.

  I know that he is working today.

• Ich denke, dass wir morgen zu Hause bleiben.

  I think that we are staying home tomorrow.

The verbs that usually come before it are worth learning as a small set: glauben (to believe/think), denken (to think), wissen (to know a fact), sagen (to say), hoffen (to hope).

English can drop "that" — I think German is interesting. German can drop dass too, and then the word order goes back to normal: Ich glaube, Deutsch ist interessant. Both are correct. Use whichever you can produce faster.

One spelling note worth having: dass with two s is the connector; das with one s is the article or which/that thing. They sound identical.