Учитель объясняет основное правило.

Because правило is a neuter singular noun, and the adjective has to agree with it in gender, number, and case.

• основной = masculine singular dictionary form
• основная = feminine singular
• основное = neuter singular

So:

• основное правило = the basic/main rule

Here both words are in the same form because they belong together as adjective + noun.

It is in the accusative case, because it is the direct object of объясняет.

The teacher explains what?
→ основное правило

However, правило is an inanimate neuter noun, so its accusative form is the same as its nominative form:

• nominative: правило
• accusative: правило

That is why it looks unchanged.

Russian nouns do change, but учитель here is the subject of the sentence, so it stays in the nominative case.

The sentence structure is:

• Учитель = subject, nominative
• объясняет = verb
• основное правило = direct object, accusative

So учитель is in its basic subject form.

It can mean either, depending on context.

Russian present tense often covers both:

• The teacher explains the basic rule.
• The teacher is explaining the basic rule.

If you need to know whether it is a general fact, a habitual action, or something happening right now, you usually understand that from the situation or from extra words.

объяснить is the infinitive: to explain.

But in this sentence you need a conjugated verb to match the subject учитель.

The subject is he/she teacher, so the verb is in 3rd person singular:

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

So учитель объясняет means the teacher explains / is explaining.

Russian has no articles like the or a/an.

So учитель can mean:

• a teacher
• the teacher

And основное правило can mean:

• a basic rule
• the basic rule

Context tells you which one is meant. This is very normal in Russian.

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

The neutral order here is:

• Учитель объясняет основное правило.

But you could also say:

• Основное правило объясняет учитель.
• Учитель основное правило объясняет.

These versions may sound more marked or give different emphasis, but the basic meaning stays the same because the endings still show who is doing the action and what is being explained.

Grammatically, учитель is a masculine noun, but in real usage it can sometimes refer to a teacher in general, depending on context.

More specifically:

• учитель = male teacher
• учительница = female teacher

In modern Russian, speakers may choose one or the other depending on style, region, and how specifically they want to identify the person.

In this sentence alone, many learners will naturally understand учитель as a male teacher, unless context says otherwise.

The stress is:

• учИтель
• объяснЯет
• основнОе
• прАвило

So the full sentence is pronounced approximately as:

uchEEtyel' obyasNYAyet asnavNOye PRAvila

That is only an approximation in English letters, but the important thing is the stressed syllables.

Because both words are neuter singular inanimate, and in Russian that usually means:

• nominative = accusative

So:

• nominative: основное правило
• accusative: основное правило

If the object were animate or of a different type, you might see a more obvious change. But here the form stays the same even though the case is accusative.

It can mean basic, main, principal, or fundamental, depending on context.

In many learning contexts, основное правило is best understood as:

• the basic rule
• the main rule

Russian adjectives often cover a range that does not match one single English word exactly, so the best translation depends on the situation.