Learn Dutch: Het weer is slecht en het regent. - The weather is bad and it is raining.

No, it does not! The word en (and) is a coordinating conjunction, which means it just glues two normal sentences together. The second part keeps the standard subject-verb order (het regent), just as if it were a standalone sentence.

Why does het regent translate to "it is raining" instead of "it rains"?

Dutch generally does not use an "is -ing" form like English does. The simple present tense het regent translates to both "it rains" and "it is raining". You can tell from the context that it is describing something happening right now.

Why is the adjective just slecht and not slechte?

When an adjective comes after the verb to describe the subject, it never gets an extra -e at the end. You only add the -e when the adjective comes directly before a noun, such as in the phrase het slechte weer (the bad weather).

Do both instances of het mean the same thing in this sentence?

No, they do two different jobs. The first het means "the" and goes with the noun weer (weather). The second het means "it", acting as the subject for the verb regent (rains), exactly like how we say "it is raining" in English.