Learn Russian: В автобусе было много пассажиров.

Пассажиров is the genitive plural of the masculine noun пассажир (passenger). Because пассажир ends in a hard consonant, we add the ending -ов to form the genitive plural. This is exactly what your current lesson is covering!

Why is the genitive case used for the passengers here?

The word много (many or a lot) always forces the noun that comes right after it into the genitive case. Since we are talking about multiple passengers, we use the genitive plural.

Why is the verb было in the neuter form if there are multiple passengers?

This is a common tricky rule! When a sentence uses a quantity word like много (many) or мало (few) as the main subject, the past tense verb defaults to the neuter singular form (было). It does not become plural to match the passengers.

Just to review, why is it в автобусе and not just в автобус?

В автобусе is in the prepositional case because it describes a static location answering the question 'where?'. If the sentence was about movement into the bus (answering 'to where?'), we would use the accusative case в автобус.