Поэтапная оплата зависит от срока сдачи.

Because поэтапная is an adjective, and it modifies the noun оплата.

• оплата = payment
• поэтапный = stage-by-stage, done in stages

So поэтапная оплата literally means stage-by-stage payment or payment in stages.

Поэтапно is an adverb, so it would describe a verb, not a noun. For example:

• Работа выполняется поэтапно. = The work is done in stages.

But before a noun like оплата, you need the adjective form: поэтапная.

Because it has to agree with оплата, which is:

• feminine
• singular
• nominative

Russian adjectives match the noun they describe in gender, number, and case.

Here:

• оплата is feminine singular nominative
• so the adjective must also be feminine singular nominative: поэтапная

Compare:

• поэтапная оплата — feminine
• поэтапный план — masculine
• поэтапное выполнение — neuter
• поэтапные выплаты — plural

Because the subject of the sentence is оплата, which is singular.

The full subject is поэтапная оплата, but the head noun is still оплата, so the verb agrees with it:

• оплата зависит = payment depends

If the subject were plural, the verb would be plural too:

• поэтапные выплаты зависят... = staged payments depend...

So зависит is singular because the subject is singular.

Because the Russian verb зависеть normally takes the preposition от.

The pattern is:

• зависеть от + Genitive

So:

• зависит от срока = depends on the deadline/term

This is just the standard construction in Russian, like English depend on.

Examples:

• Это зависит от цены. = It depends on the price.
• Результат зависит от опыта. = The result depends on experience.

So after зависеть, you usually expect от.

Because от requires the genitive case here.

The noun срок becomes срока in the genitive singular:

• nominative: срок
• genitive: срока

Since the pattern is зависеть от + Genitive, we get:

• зависит от срока

This is one of the most common things learners need to remember: many prepositions force a specific case, and от commonly takes the genitive.

Because сдача depends on срок in the phrase срок сдачи.

This is a very common Russian pattern:

• noun + noun in the genitive
• literally: deadline of submission/delivery/completion

So:

• срок = term, deadline
• сдачи = of submission / of handover / of completion

Together:

• срок сдачи = submission deadline / completion deadline / delivery deadline

Then the whole phrase goes after от, so you get:

• от срока сдачи

You can think of it like nested structure:

• зависит от
  • срока сдачи
• inside that, срока is governed by от
• сдачи is governed by срока

In this sentence, сдача most likely means something like:

• delivery
• submission
• handover
• completion and handover

The exact English choice depends on context.

The noun сдача comes from the verb сдать / сдавать, which can mean different things in different situations:

• сдать экзамен — to pass/take an exam
• сдать работу — to submit work
• сдать проект — to deliver/hand in a project
• сдать объект — to complete and hand over a construction site/building

So срок сдачи is a very common business/project phrase meaning the deadline by which something must be handed in, delivered, or completed.

Yes, they are very close in meaning.

• поэтапная оплата = stage-by-stage payment
• оплата по этапам = payment by stages / payment in stages

The difference is mostly stylistic:

• поэтапная оплата sounds compact and formal
• оплата по этапам sounds a little more explicit and conversational

Both are natural, especially in business or contract language.

Yes, Russian word order is flexible, though not completely free.

The neutral order here is:

• Поэтапная оплата зависит от срока сдачи.

But you could also say:

• От срока сдачи зависит поэтапная оплата.

That version puts more emphasis on от срока сдачи — the factor the payment depends on.

In formal writing, the original order is usually the most straightforward and natural.

A natural stress pattern is:

• поэта́пная опла́та зави́сит от сро́ка сда́чи

Helpful notes:

• поэта́пная — stress on та
• опла́та — stress on ла
• зави́сит — stress on the ви
• сро́ка — stress on сро
• сда́чи — stress on да

A rough pronunciation guide:

• pah-ee-TAP-na-ya ap-LA-ta za-VEE-sit at SRO-ka SDA-chee

Not perfect English-style transcription, but enough to help with stress placement.

Yes, it sounds fairly formal and business-like.

Words and phrases like:

• поэтапная оплата
• срок сдачи

are very common in:

• contracts
• project planning
• construction
• freelance agreements
• procurement and business correspondence

In everyday conversation, a speaker might choose something simpler depending on context, but this sentence is completely normal in professional Russian.