Я сожалею, что не смог прийти вовремя.

Что introduces a clause meaning that:

• Я сожалею = I regret / I am sorry
• что не смог прийти вовремя = that I couldn’t come on time

So the structure is very similar to English I regret that...

In Russian, что is the normal connector here. Unlike English, where that is often omitted, Russian usually keeps что in sentences like this.

Because что не смог прийти вовремя is a subordinate clause.

Russian normally puts a comma before conjunctions like что when they introduce a full clause:

• Я сожалею, что...
• Я думаю, что...
• Я знаю, что...

So the comma is required here.

This is about aspect and nuance.

• не смог = was not able to / failed to
• не мог = could not / was unable to, often sounding more ongoing, situational, or descriptive

Here the speaker is talking about one specific completed situation: there was an intended act of arriving on time, and it did not happen. That is why не смог sounds natural.

Compare:

• Я не смог прийти вовремя. = I wasn’t able to come on time.
• Я не мог прийти вовремя, потому что все дороги были закрыты. = I couldn’t come on time because all the roads were closed.

Both can work in some contexts, but не смог strongly fits the idea of a single failed result.

Because the regret is happening now, but the missed action happened in the past.

• сожалею = I regret / I am sorry now
• не смог прийти = I was not able to come earlier

This is completely normal in both Russian and English: present feeling about a past event.

Because the sentence refers to something that already happened.

• могу = I can
• смогу = I will be able to
• смог = I was able to / managed to

Since the speaker is regretting a past failure to arrive on time, Russian uses the past form смог with negation: не смог.

Смог is the masculine singular past tense form of the verb смочь.

Forms:

• masculine: смог
• feminine: смогла
• neuter: смогло
• plural: смогли

So if the speaker is male, смог is correct. If the speaker is female, it would be:

• Я сожалею, что не смогла прийти вовремя.

Russian often omits a subject pronoun when it is already clear from context.

So:

• Я сожалею, что не смог прийти вовремя.

means the same as:

• Я сожалею, что я не смог прийти вовремя.

The second version is possible, but repeating я is usually unnecessary unless you want emphasis or contrast.

Again, this is an aspect choice.

• прийти = perfective, to arrive / to come (as a single completed event)
• приходить = imperfective, to come / be coming / come regularly

The sentence is about one specific occasion: the speaker failed to arrive on time once. That calls for прийти.

Compare:

• не смог прийти вовремя = couldn’t arrive on time
• не мог приходить вовремя = couldn’t keep coming on time / couldn’t come on time regularly

So прийти is the natural choice here.

Вовремя is an adverb meaning on time or at the right time.

It is written as one word.

Be careful not to confuse it with во время, which is usually a preposition + noun meaning during:

• вовремя = on time
• во время урока = during the lesson

So in this sentence:

• прийти вовремя = to arrive on time

Yes. Russian word order is flexible.

The version given:

• Я сожалею, что не смог прийти вовремя.

is neutral and natural.

You could also hear:

• Я сожалею, что не смог вовремя прийти.

This is still understandable, but it sounds a little less neutral. In many contexts, placing вовремя at the end sounds smoother.

Russian often changes word order for emphasis, rhythm, or style, not because the grammar changes.

Yes, it is grammatical and understandable, but it can sound a bit formal or bookish in everyday conversation.

Very common alternatives are:

• Мне жаль, что не смог прийти вовремя. = I’m sorry that I couldn’t come on time.
• Извините, что не смог прийти вовремя. = Sorry that I couldn’t come on time.
• Простите, что не смог прийти вовремя. = Forgive me / Sorry that I couldn’t come on time.

So Я сожалею is fine, but in casual spoken Russian, many people would more naturally say Мне жаль or Извините depending on the situation.

Yes, сожалеть / сожалею can be followed by different structures.

Common ones include:

• сожалеть, что... = to regret that...
• сожалеть о чём-то = to regret something / feel sorry about something

Examples:

• Я сожалею, что опоздал. = I regret that I was late.
• Я сожалею об этом. = I regret that / I’m sorry about that.

In your sentence, что + clause is used because the speaker is regretting a whole event, not just naming a thing.