Mostly the words have the same Gender as in German. But it's not that important. If you do not know, or do not care, just use 'hij' where in English you would use 'it'.
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We don't care, really, apart from some language puritans. The genders are 'leftovers' from the time that German and Dutch were a dialect continuum. (Say 1000 until the year 1850.) Dutch moved away from gender and grammatical cases. Actually in 1948 there was a reform that officially stated the Gender of words are not necessary in official correct Dutch anymore, and neither was the Akkusativ grammatical case. I hope in the following centuries, we abolish the difference between 'de' and 'het'. It would make the language more logical.