German Grammar Explained /

Conjunction: dass

Ich glaube, dass er morgen kommt.
I think that he is coming tomorrow.
Since dass is a conjunction introducing a sub clause, you need to put the verb at the end. You also need a comma to separate both clauses. German is very strict on separating clauses!
A difference to notice between German and English is that in English we avoid "that" whenever possible.
Ich habe Angst, dass wir den Flug verpassen.
I'm worried that we will miss the flight.
You could leave dass out...
Ich habe Angst, wir verpassen den Flug.
I'm worried we will miss the flight.
But this is just two main clauses (mind the verb) joined together by a comma. This omission is more common in English. In German it is only used in colloquial or spoken language.