Learn Russian: Нам интересно, а им скучно.

In Russian, expressing a physical or emotional state like being interested or bored uses an impersonal structure. Instead of saying 'We are interested', you literally express 'To us it is interesting'. Нам (to us) is the dative case of мы, and им (to them) is the dative case of они.

Why do интересно and скучно end in -о?

These are state words (predicative adverbs) used in impersonal sentences. Because there is no grammatical subject like 'he' or 'she' performing an action, these words always take this default -о ending, regardless of who is experiencing the feeling.

The English translation says 'and', so why does the Russian sentence use а instead of и?

The conjunction а is used to show a contrast between two things. Because the sentence contrasts two opposing states (we are interested while they are bored), а is the natural choice. It often translates to 'and' or 'but' in English depending on what sounds best.

Why is there no verb for 'are' in this sentence?

In the present tense, Russian leaves out the 'to be' verb (is/are). If you wanted to talk about the past and say 'We were interested', you would need to add the neuter past tense verb было, making it нам было интересно.