На аллее было много жёлтых листьев.

Because на here means on / in in the sense of location, and after на with a location meaning, Russian normally uses the prepositional case.

• аллея = nominative singular
• на аллее = prepositional singular

So:

• аллея = an alley / a tree-lined path
• на аллее = on the alley / on the path

This is the same pattern as:

• в парке = in the park
• на улице = on the street

Аллея is a feminine noun ending in -я. In the prepositional singular, nouns like this usually change to -е.

So:

• аллея → nominative
• на аллее → prepositional

This is a very common pattern:

• неделя → на неделе
• аудитория → в аудитории (slightly different pattern because of spelling and noun type)
• аллея → на аллее

This is one of the most common things learners ask about this kind of sentence.

Russian often uses neuter singular past tense of быть in existential sentences like there was / there were, especially when the main noun phrase is introduced by a quantity word such as много.

So:

• На аллее было много жёлтых листьев. = There were a lot of yellow leaves on the path.

Even though leaves is plural in meaning, the structure with много normally uses было.

Compare:

• На аллее были жёлтые листья. = There were yellow leaves on the path.
  • Here the grammatical subject is just жёлтые листья, so были is used.

But:

• На аллее было много жёлтых листьев.
  • With много, Russian usually uses было.

Много means many / a lot of.

It is a quantity word, and in Russian quantity words often affect the grammar of the noun that follows. After много, the noun normally goes into the genitive plural.

So:

• листья = leaves (nominative plural)
• много листьев = many leaves / a lot of leaves (genitive plural)

This is similar to:

• много людей = many people
• много книг = many books
• много домов = many houses

Because after много, the noun phrase goes into the genitive plural.

That means both words change:

• жёлтые листья = yellow leaves (nominative plural)
• жёлтых листьев = of yellow leaves / yellow leaves after много (genitive plural)

The adjective has to agree with the noun in case and number, so:

• жёлтые → nominative plural
• жёлтых → genitive plural

And:

• листья → nominative plural
• листьев → genitive plural

This noun has an irregular-looking plural pattern that is very common in Russian.

Basic forms:

• singular: лист = leaf
• nominative plural: листья = leaves
• genitive plural: листьев

So in this sentence, after много, you need the genitive plural:

• много листьев

Many Russian neuter-style plural forms in -ья behave like this:

• дерево → деревья → деревьев
• крыло → крылья → крыльев
• лист → листья → листьев

So this is a pattern worth memorizing.

Because Russian does not need a dummy subject like English there.

English says:

• There were a lot of yellow leaves on the path.

Russian simply says:

• На аллее было много жёлтых листьев.

The idea of existence is expressed by the whole sentence structure, not by a separate word meaning there.

This is very normal in Russian:

• В комнате был стол. = There was a table in the room.
• На улице было много людей. = There were many people outside/on the street.

In standard Russian, that would not be the normal choice.

With много + genitive plural, Russian usually prefers было, not были:

• На аллее было много жёлтых листьев. ✅

But if you remove много, then были becomes normal:

• На аллее были жёлтые листья. ✅

So a good rule for learners is:

• simple plural noun → often были
• много + noun → usually было

Because на is the natural preposition for many open outdoor spaces, surfaces, routes, and public places in Russian.

A path or alley is understood as a surface/route you are on, so Russian says:

• на аллее = on the path / along the alley

Using в would sound unnatural here, because в usually suggests being inside something enclosed.

Compare:

• в парке = in the park
• на аллее = on the path/in the alleyway
• на улице = on the street

Prepositions do not always match English one-for-one, so this is something to learn as a Russian usage pattern.

Yes, but mostly for focus and style, not basic meaning.

The sentence:

• На аллее было много жёлтых листьев.

is a very natural way to present the information. It starts with the location, then says what was there.

That structure is common in Russian existential sentences:

• Где? → Что было?
• На аллее → было много жёлтых листьев

You may also hear different word orders, but they change emphasis:

• Много жёлтых листьев было на аллее.
  • More emphasis on a lot of yellow leaves
• Жёлтых листьев на аллее было много.
  • Strong emphasis on many

For a learner, the original sentence is the safest and most natural choice.

In the present tense, Russian usually omits the verb быть in this kind of sentence.

So:

• past: На аллее было много жёлтых листьев.
• present: На аллее много жёлтых листьев.

Not:

• На аллее есть много жёлтых листьев in normal neutral style

Russian often has zero copula in the present, especially in simple descriptive/existential sentences.

So the present-tense version is simply:

• На аллее много жёлтых листьев.

Grammatically, было is the neuter singular past form of быть.

But in English, the best translation is often there were, because the meaning involves a plural quantity:

• На аллее было много жёлтых листьев. = There were a lot of yellow leaves on the path.

So the form is singular in Russian grammar, but the natural English translation is plural.

This mismatch is completely normal and is caused by the Russian construction with много.

It is genitive plural.

You can identify it like this:

• много requires the genitive
• because the meaning is plural (many leaves), it becomes genitive plural

So:

• жёлтые листья = nominative plural
• жёлтых листьев = genitive plural

Both words are plural, and both are in the genitive.

This is one of the most important patterns in Russian:

• много + genitive plural
• мало + genitive plural
• несколько + genitive plural

Examples:

• много книг
• мало людей
• несколько домов