Yeah In Hindi, there are no articles being used thus it always means definite unlike English where definite sentences are brought into alert by the usage of the. But in Hindi the sentences naturally means definite case . Thus it makes sense to add The rats are black.( Not Rats are black.) And I think the Hindi translation for Rats are black चुहे काले होते हैं Forgive me if there are mistakes, I'm a learner too.