Он совершил серьёзную ошибку.

Совершил is the past tense of совершить, so the sentence is talking about something that already happened.

Why not совершает?

• совершает = is committing / commits
• That would make the sentence present tense: He is committing a serious mistake or He commits a serious mistake

Why not сделал?

• сделать means to do / make, and in some contexts it can work with ошибка, especially in the phrase сделать ошибку, but совершить ошибку is a very common and natural collocation in Russian, especially in slightly more formal or standard language.
• So совершил ошибку sounds like committed/made a mistake, with совершить being the usual verb here.

Here совершить means to commit or to carry out.

It is often used with nouns that describe an action or act, for example:

• совершить ошибку — to make/commit a mistake
• совершить преступление — to commit a crime
• совершить поездку — to make/take a trip

So in this sentence, it does not mean something like to perfect or to complete oneself. It simply forms a natural expression with ошибка.

Because ошибка is the direct object of the verb.

The basic form is:

• ошибка — mistake (nominative singular)

After a transitive verb like совершил, the direct object usually goes into the accusative case:

• ошибку — accusative singular

Since ошибка is a feminine noun ending in -а, its accusative singular usually changes:

• -а → -у

So:

• ошибка → ошибку

Because the adjective has to agree with the noun it describes.

The noun is:

• ошибка — feminine singular
• in this sentence it is in the accusative singular: ошибку

So the adjective must match:

• feminine
• singular
• accusative

That gives:

• серьёзная ошибка — nominative
• серьёзную ошибку — accusative

This is normal adjective agreement in Russian.

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

The infinitive is:

• совершить

The past tense stem gives:

• совершил — he committed
• совершила — she committed
• совершило — it committed
• совершили — they committed

So -л here is part of the past tense form, and the zero ending after it shows masculine singular.

Because the subject is он (he).

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

• он совершил
• она совершила
• оно совершило
• они совершили

So the form совершил tells you the subject is masculine singular.

Yes. Совершить is a perfective verb.

That means it presents the action as:

• completed
• whole
• a single event

So Он совершил серьёзную ошибку means he did make / ended up making a serious mistake as a completed fact.

The imperfective partner is совершать:

• Он совершал ошибки — He was making mistakes / He used to make mistakes
• Он совершил ошибку — He made a mistake (one completed act)

This is a very common aspect contrast in Russian.

Yes, often it can.

Russian frequently drops subject pronouns when the meaning is already clear from context:

• Совершил серьёзную ошибку.

This can still mean He made a serious mistake, if the subject is understood from the situation.

But including он can be useful:

• to make the subject explicit
• to emphasize he
• to contrast him with someone else

So both are possible, depending on context.

Yes. Russian word order is flexible, although the neutral order here is very natural:

• Он совершил серьёзную ошибку.

Other orders are possible for emphasis:

• Серьёзную ошибку он совершил. — emphasizes the serious mistake
• Ошибку он совершил серьёзную. — possible, but more marked
• Совершил он серьёзную ошибку. — can sound more narrative or expressive

For learners, the given order is the safest and most neutral one.

The stress is:

• Он
• соверши́л
• серьёзную
• оши́бку

A few useful notes:

• In соверши́л, the stress is on -ши́л
• In оши́бку, the stress is on ши́
• In серьёзную, the letter ё is always stressed: серьёзную

That makes ё especially helpful for pronunciation.

Not always in everyday writing.

You may see:

• серьёзную
• серьезную

Both usually mean the same word, but ё is the more precise spelling because it shows pronunciation clearly.

In printed learning materials, dictionaries, and careful writing, ё is often kept. In casual writing, it is often replaced with е, and the reader is expected to know the word.

Yes, very common.

Russian often combines:

• совершить ошибку
• допустить ошибку
• сделать ошибку (less standard in some contexts, though understandable)

And adding серьёзную simply makes the mistake more serious:

• ошибка — a mistake
• серьёзная ошибка — a serious mistake

So the whole sentence sounds natural and standard.