Learn Russian: Горькое лекарство — это плохо.

In Russian, the present tense verb for is is generally skipped. When making a broad statement or defining a concept, Russian often uses a long dash followed by это to connect the subject (горькое лекарство) to its description. It acts a bit like saying Bitter medicine, this is a bad thing.

Why does the sentence use the adverb плохо instead of the neuter adjective плохое?

This is a special rule for statements using это. When you use это to make a general statement about a situation or concept, the descriptive word that follows is usually an adverb ending in -о, like плохо. If you were describing a specific medicine without это, you would use the matching neuter adjective: горькое лекарство плохое.

Why does the adjective горькое end in -ое?

The ending changes to match the gender of the noun it describes. The noun лекарство (medicine) ends in -о, which tells us it is a neuter word. To match it, the dictionary form of the adjective горький (bitter) drops its usual ending and takes the neuter ending -ое to become горькое.