"I see a bird on the roof."
Vertaling:Ik zie een vogel op het dak.
I'll assume you speak English.
You must use 'de' with common gender words (the so-called de-woorden), and 'het' with neuter gender nouns (het-woorden).
Unfortunately there are no rules we can follow in order to figure out what the gender of a noun is, but some very general guidelines (like 'all plural nouns go with de' or 'all diminutives go with het').
You can find some of the guidelines on the following links:
You can also check in a good dictionary or on the following website:
There are also a few apps you can download that tell you whether a word goes with de or het.