"That is the first wedding."
Translation:Das ist die erste Hochzeit.
"Diese" means "this" as an adjective, not as a pronoun. For instance, one could say "diese Steckdose" to mean "this power outlet", but one would have to use "das" to say "this is a power outlet" ("das ist eine Steckdose").
yes, that's german but if you thought there were only three, oh boy are you in for a treat. https://www.google.co.uk/search?q=german+case+table=1CAACAC_enGB637GB637=2=1366=655=lnms=isch=X=0ahUKEwisjtmYq4TOAhViLsAKHVYuChYQ_AUIBygC#imgrc=pUj7bFby0KXZcM%3A