"The old man is writing his first book."
Translation:De oude man schrijft zijn eerste boek.
In addition to what xMerrie says: Because 'his' is a possessive pronoun, for which an adjective would also end like if there were a definite article:
het mooie boek
zijn mooie boek
een mooi boek
geen mooi boek
de mooie kast
zijn mooie kast
een mooie kast
geen mooie kast
'Eerste' is an ordinal number (so 1st, 2nd, 3rd, 4th, etc). For these words, the endings do not change. :)
'Eerst' is also a word though. It also means first, but in a more timeline-way. As in 'first I am going to do this, and then I'll do that'.
I still dont understand when to use het and when de, I thought that het is for single and de is for plural but it doaent always apply. Can someone please explain?