Мы ждали автобуса впустую, потому что он не приехал.

Because ждать often takes the genitive in Russian: ждать автобуса, ждать друга, ждать ответа.

So in this sentence:

• автобус = nominative
• автобуса = genitive singular

A useful learner rule is: ждать does not always behave like a normal English-style direct-object verb.

Also, in modern Russian, you may hear ждать автобус too. Both are possible in many contexts, but ждать автобуса is very common and completely standard.

Ждали already does mean waited / were waiting, depending on context.

It is the past tense plural form of ждать:

• я ждал / ждала
• ты ждал / ждала
• он ждал
• она ждала
• мы ждали
• они ждали

A very important thing to notice: in the Russian past tense, the verb agrees with gender and number, not person. So with мы, you use the plural ending -ли: ждали.

Because the two actions are being viewed differently.

• ждали is imperfective because waiting is presented as a process / ongoing situation
• приехал is perfective because arriving is seen as a single completed event

So the sentence presents this contrast:

• We were waiting / waited in vain
• because the bus never arrived

With negation, не приехал means did not arrive or failed to arrive.

If you used не приезжал, that would usually suggest a different idea, such as repeated or non-specific coming, not this one expected arrival that never happened.

Because автобус is a masculine noun, and Russian pronouns match the grammatical gender of the noun.

So:

• автобус → masculine
• therefore он

This is normal in Russian even for inanimate things. English usually says it, but Russian uses the pronoun that matches the noun’s gender:

• автобус → он
• машина → она
• окно → оно

So он не приехал literally uses he, but in natural English you translate it as it didn’t arrive.

Because приехать is the normal verb for arriving by vehicle / transport.

A bus does not walk somewhere, so пришёл would not fit.

Very roughly:

• прийти = come on foot / arrive
• приехать = come by transport / arrive by vehicle

Since a bus is a vehicle, приехал is exactly what you would expect.

Впустую is an adverb meaning:

• in vain
• for nothing
• to no purpose
• uselessly

So ждали автобуса впустую means that the waiting was pointless because the bus never came.

It does not mean physical emptiness here. It is about wasted effort or wasted time.

A close synonym is напрасно, though the tone and usage can vary a little depending on context.

No. Впустую is just an adverb.

That means it does not change for case, gender, or number. It simply modifies the action ждали.

So in this sentence, it works like an English adverb in waited in vain.

Because Russian has no articles.

English must choose between:

• a bus
• the bus

Russian usually leaves that unstated and lets the context show it.

So автобуса can mean:

• a bus
• the bus

In this sentence, the context clearly suggests the bus: the speakers were waiting for some expected bus, and it did not arrive.

Yes. Russian word order is much more flexible than English word order.

All of these are possible, with small differences in emphasis:

• Мы ждали автобуса впустую
• Мы впустую ждали автобуса
• Автобуса мы ждали впустую

The version you have is natural and neutral. If you move впустую earlier, you often emphasize the idea of it was all for nothing a bit more strongly.

So the meaning stays basically the same, but the focus can shift.

Sometimes, especially in conversation, Russian can omit something that is obvious from context. So you might hear something shorter in speech.

But in a clear, standard sentence, он is very natural here because it makes the subject of не приехал explicit: the bus.

So:

• потому что он не приехал = clear and standard
• потому что не приехал = possible in context, but more dependent on the listener already knowing who or what did not arrive

Потому что is a very common way to say because in Russian.

So the structure is:

• main clause: Мы ждали автобуса впустую
• reason clause: потому что он не приехал

It is one of the most useful and neutral connectors for giving a reason. English speakers can often translate it directly as because.