Субподрядчик установил утеплитель и строительные леса.

The sentence breaks down like this:

• Субподрядчик = the subject (the subcontractor)
• установил = the verb (installed / set up)
• утеплитель и строительные леса = the direct objects (insulation and scaffolding)

So the overall pattern is a very standard Russian subject + verb + object sentence.

In the past tense, Russian verbs agree with the subject in gender and number.

• установил = masculine singular
• установила = feminine singular
• установили = plural

Since субподрядчик is a grammatically masculine singular noun, the verb appears as установил.

Установил is perfective.

That means the sentence presents the action as completed: the subcontractor finished installing / setting up the items.

The imperfective partner would be устанавливал. Compare:

• Субподрядчик установил... = the subcontractor installed / has installed (completed result)
• Субподрядчик устанавливал... = the subcontractor was installing / used to install (process, repeated action, or background information)

In this sentence, the perfective makes sense because it sounds like a completed job.

Yes, they are direct objects, so they are in the accusative case.

But in Russian, for inanimate nouns:

• masculine singular accusative often looks the same as nominative
• plural accusative often looks the same as nominative

So:

• утеплитель is masculine singular inanimate, and its accusative looks the same as its nominative
• строительные леса is plural inanimate, so the accusative also looks the same as the nominative

That is why they appear unchanged.

Russian commonly uses строительные леса as a plural expression for scaffolding.

So even though English often uses one singular-looking word, Russian treats this idea as plural:

• леса = scaffolding / scaffold structures
• строительные is plural because it agrees with леса

This is one of those places where Russian and English package the same idea differently.

Yes, леса can also mean forests in other contexts.

But here, the phrase строительные леса is a standard construction term meaning scaffolding.

So context tells you which meaning is intended:

• густые леса = dense forests
• строительные леса = scaffolding

This is a good example of why it helps to learn some noun combinations as set phrases.

Because и simply joins two direct objects in one straightforward list:

• утеплитель
• строительные леса

In Russian, just like in English, you normally do not put a comma before и when it connects two items like this.

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

The version here:

• Субподрядчик установил утеплитель и строительные леса.

is the most neutral, straightforward order.

But other orders are possible for emphasis, topic, or contrast, for example:

• Утеплитель и строительные леса установил субподрядчик.
  This puts more focus on what was installed.

Even when the word order changes, the cases still show who did what.

Russian has no articles.

So субподрядчик can mean:

• a subcontractor
• the subcontractor

And the same goes for the objects.

Russian usually leaves definiteness to be understood from:

• context
• word order
• previous mention
• general situation

So the sentence itself does not explicitly mark a vs the.

Yes, it works.

With утеплитель, установить means something like:

• install
• put in
• fit

With строительные леса, it means:

• set up
• erect

Russian often uses one verb with two coordinated objects if the general idea fits both. In technical writing, a speaker might choose a more specific verb for one of the items, but this sentence is perfectly understandable and natural in a construction context.