Он понял суть вопроса.

• Он = he
• понял = understood / realized / grasped
• суть = the essence / the core / the main point
• вопроса = of the question / of the issue

So grammatically:

• Он is the subject.
• понял is the verb.
• суть вопроса is the object phrase, literally the essence of the question.

This is a very common aspect question.

• понял is perfective
• понимал is imperfective

Here, понял shows a completed result: he came to understand it, he got the point.

So:

• Он понял суть вопроса = he understood the point / he grasped the essence
• Он понимал суть вопроса = he understood the point in a more ongoing or state-like sense, depending on context

In this sentence, the speaker is focusing on the moment or fact of successful understanding, so понял is the natural choice.

In Russian, the past tense is usually formed with -л.

The base infinitive is понять. Its past tense forms are:

• он понял = he understood
• она поняла = she understood
• оно поняло = it understood
• они поняли = they understood

So -л marks the past tense, and in понял the form is masculine singular because it agrees with он.

Суть means essence, core, or main point.

So суть вопроса is not just the literal wording of the question. It means:

• the central issue
• the real point
• what the question is really about

This is slightly more abstract and formal than simply talking about the question itself.

For example:

• Он понял вопрос = he understood the question
• Он понял суть вопроса = he understood the real point behind the question

Because вопроса is in the genitive case.

In суть вопроса, the second noun answers essence of what?

• суть чего? = the essence of what?
• вопроса = of the question

This is a very common Russian pattern:

• начало книги = the beginning of the book
• конец фильма = the end of the film
• центр города = the center of the city
• суть вопроса = the essence of the question

So вопроса is genitive singular of вопрос.

Here суть is in the accusative case because it is the direct object of понял.

However, an important detail is that суть is a feminine noun, and for this noun the nominative and accusative forms are the same:

• nominative: суть
• accusative: суть

So even though the form does not change, its role in the sentence is accusative.

Yes, very often.

Russian often leaves out subject pronouns when the meaning is clear from context. So you could simply say:

• Понял суть вопроса.

That can mean He understood the point if the subject is already known.

Including он makes the subject more explicit. It may be used:

• to introduce him as the subject
• to contrast him with someone else
• to add emphasis

For example:

• Она не поняла, а он понял суть вопроса.
  She didn’t understand, but he understood the point.

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

The neutral order here is:

• Он понял суть вопроса.

But other orders are possible, depending on emphasis:

• Суть вопроса он понял.
  Emphasis on the essence of the question
• Понял он суть вопроса.
  Can sound marked, contrastive, or emotionally colored depending on context

Even when the order changes, the cases help show what each word is doing.

Russian has no articles like a or the.

Whether something is definite or indefinite is understood from:

• context
• word order
• stress/emphasis
• general meaning

So суть вопроса can naturally mean the essence of the question without any separate word for the.

This is normal in Russian:

• Я читаю книгу = I am reading a/the book
• Он понял суть вопроса = He understood the essence of the question

Yes, usually a little more formal or abstract.

Compare:

• Он понял вопрос. = He understood the question.
• Он понял суть вопроса. = He understood the essence / real point of the question.

The second version suggests deeper understanding. It is common in:

• formal speech
• academic or professional contexts
• discussions where the main issue matters more than the exact wording

The stress is:

• Он понЯл сУть вопрОса.

More precisely:

• он
• понЯл
• сУть
• вопрОса

A rough English-style pronunciation guide:

• on pa-NYAL soot' va-PRO-sa

A few notes:

• я in понял is stressed
• суть ends in a soft consonant
• вопроса has stress on -ро-

Суть is a feminine noun, which can surprise learners because it does not look like the very common -а / -я feminine pattern.

For example:

• nominative: суть
• genitive: сути
• dative: сути
• accusative: суть
• instrumental: сутью
• prepositional: сути

So it is a real, normal noun, but its pattern is less basic than nouns like книга or машина. That is one reason learners often notice it in this sentence.