Learn Russian: Завтрак включён в счёт, поэтому вам не надо тратить деньги. - Breakfast is included in the bill, so you do not need to spend money.

Включён is a short-form past passive participle meaning "included." It comes from the perfective verb включить (to include). Because the subject завтрак is a masculine noun, the participle takes the masculine short-form ending with no extra vowels at the end.

Since the breakfast is in the bill, why is it в счёт (accusative) instead of в счёте (prepositional)?

The verb включить (to include) takes в + accusative case because the action involves adding something into a list or total. Even though in English we say "in the bill," Russian treats this addition as a direction or target, requiring the accusative счёт.

Why does the second part of the sentence use the dative case вам instead of вы?

Words like надо and нужно create impersonal sentences. The person who needs to do the action is put in the dative case (вам). You can think of it literally as "for you, it is not necessary."

Why do we use the imperfective verb тратить (to spend) rather than the perfective потратить?

When you want to express that there is no need to do something using не надо or не нужно, you almost always use the imperfective infinitive. Using the imperfective emphasizes that engaging in the general action of spending money is unnecessary.