Учёные проверили теорию.

Here учёные is a substantivized adjective: literally, it comes from the adjective учёный (learned, scientific), but in real usage it often means scientist.

So:

• учёный = a scientist / learned (masculine singular)
• учёные = scientists (plural)

In this sentence, it functions as a noun: The scientists tested the theory.

Учёные is in the nominative plural.

Why?

• It is the subject of the sentence: the people doing the action.
• In Russian, the subject is usually in the nominative case.

So:

• учёный = nominative singular
• учёные = nominative plural

Because теорию is in the accusative singular.

In this sentence, theory is the direct object: it is the thing being tested. Russian marks that with case.

For a feminine noun ending in -я, the accusative singular usually changes to -ю:

• теория = nominative singular
• теорию = accusative singular

So:

• Учёные = the scientists
• проверили теорию = tested the theory

Проверили is perfective past plural, while проверяли would be imperfective past plural.

The difference is aspect:

• проверили = they checked/tested it, viewed as a completed whole
• проверяли = they were checking / used to check / checked with focus on the process or repeated action

In a simple sentence like this, проверили usually means the action was completed: they carried out the test/check.

In the Russian past tense, plural verbs usually end in -и:

• masculine singular: проверил
• feminine singular: проверила
• neuter singular: проверило
• plural: проверили

Since the subject is учёные (scientists, plural), the verb is also plural: проверили.

The stress is:

• учёные → u-CHO-ny-ye
• проверили → pra-VYE-ri-li
• теорию → TYE-o-ri-yu

A more careful breakdown:

• учёные = u-CHYO-ny-ye
• проверили = pra-VYE-ri-li
• теорию = TYE-o-ri-yu

The letter ё is always stressed.

Yes, in many Russian texts ё is often written as е, so you may see ученые instead of учёные.

But the pronunciation is still ё here, not plain е.

So:

• written carefully: учёные
• often written in ordinary text: ученые

For learners, it is very helpful to remember that ё:

• is always stressed
• changes pronunciation
• can distinguish words or forms

Yes. Russian word order is more flexible than English because case endings show the grammatical roles.

Basic neutral order:

• Учёные проверили теорию.

Other possible orders:

• Теорию проверили учёные.
• Проверили учёные теорию.

These versions usually change the focus or emphasis, not the core meaning.

For example:

• Учёные проверили теорию. = neutral
• Теорию проверили учёные. = emphasizes the theory
• Проверили учёные теорию. = can sound more marked or context-dependent

Russian has no articles like a or the.

So Учёные проверили теорию can mean:

• Scientists tested the theory
• The scientists tested the theory

Which one is intended depends on the context.

It can be either, depending on context.

The verb проверить has a broad meaning:

• to check
• to verify
• to test

With теорию (theory), English often prefers:

• tested the theory
• sometimes checked the theory
• sometimes verified the theory, depending on the situation

So the Russian verb is slightly broader than any one single English word.