Эта теория объясняет результат.

Because эта must agree with теория in gender, number, and case.

• теория is feminine
• it is singular
• here it is in the nominative case because it is the subject of the sentence

So the correct form of this is эта.

Compare:

• этот результат = this result
• эта теория = this theory
• это объяснение = this explanation

In Russian, nouns ending in -а or -я are usually feminine, and теория ends in -я.

That is why it takes feminine agreement:

• эта теория
• новая теория
• интересная теория

This is one of the most common noun patterns in Russian.

Объясняет is the 3rd person singular form of the verb объяснять.

The subject is эта теория, which means it is:

• third person = not I or you
• singular = one theory

So the verb must match that subject:

• я объясняю = I explain
• ты объясняешь = you explain
• он / она объясняет = he / she explains
• эта теория объясняет = this theory explains

Russian often uses the present tense for general truths, statements, and descriptions, just like English does.

So эта теория объясняет результат can express a general idea: this theory explains the result.

The present tense here does not have to mean right now at this exact moment. It can also mean:

• generally
• typically
• as a fact or claim

Объясняет comes from the imperfective verb объяснять.

Russian verbs have aspect, and here the imperfective is natural because the sentence presents the action as:

• a general fact
• an ongoing explanatory relationship
• not focused on a single completed act

If you used the perfective verb объяснить, its future form would be объяснит, which would sound more like:

• it will explain
• it will manage to explain
• it will account for it in a completed sense

So in a neutral statement like this, объясняет is the expected choice.

It actually is functioning as the direct object, so it is in the accusative case. But for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: результат
• accusative: результат

That is why there is no visible ending change here.

Because it is the thing being explained. In Russian, the direct object of a verb like объяснять normally goes in the accusative case.

So the structure is:

• Эта теория = subject
• объясняет = verb
• результат = direct object

A learner might expect a preposition, but Russian often uses a simple verb + accusative pattern where English does too.

Russian has no articles. There is no direct equivalent of a, an, or the.

So теория can mean:

• a theory
• the theory

And результат can mean:

• a result
• the result

The exact meaning depends on context, not on an article.

No, Russian word order is fairly flexible, because case endings help show who does what.

Эта теория объясняет результат is the most neutral order here:

• subject → verb → object

But other orders are possible, with different emphasis. For example:

• Результат объясняет эта теория
  Emphasizes this theory as the one that explains the result.

• Объясняет эта теория результат
  Possible, but less neutral and more marked in style or emphasis.

So the original sentence is the most natural basic version.

The stress is:

• Э́та
• тео́рия
• объясня́ет
• результа́т

A rough pronunciation guide:

• Э́та = EH-ta
• тео́рия = tee-O-ri-ya
• объясня́ет = ab-yis-NYA-yet
• результа́т = ree-zool-TAT

The stressed syllable is very important in Russian, because stress is not always predictable.

Yes, absolutely.

• Теория объясняет результат = Theory explains the result / A theory explains the result / The theory explains the result
• Эта теория объясняет результат = This theory explains the result

Adding эта makes the noun more specific. It points to a particular theory, just like this does in English.