К сожалению, я не смогу прийти сегодня вечером.

К сожалению means unfortunately or literally to regret / to [one’s] regret.

The noun is сожаление = regret. After the preposition к, Russian often uses the dative case, so сожаление becomes сожалению.

So:

• сожаление = regret
• к сожалению = unfortunately

This is a fixed expression, so it is best learned as a whole phrase.

Because К сожалению is an introductory phrase, similar to English unfortunately at the beginning of a sentence.

Russian usually separates such parenthetical or introductory expressions with commas:

• К сожалению, я не смогу прийти.
• К счастью, всё получилось. = Fortunately, everything worked out.

So the comma is standard here.

Не могу means I can’t / I am not able to in the present.

Не смогу means I won’t be able to in the future.

Since the sentence refers to this evening, the speaker is talking about a future situation, so Russian uses:

• я не могу прийти = I can’t come
• я не смогу прийти = I won’t be able to come

So не смогу is the correct choice for I won’t be able to come tonight.

Смогу is the 1st person singular future form of смочь, which means to be able to / to manage to.

The forms are irregular, so it helps to memorize them:

• я смогу = I will be able
• ты сможешь = you will be able
• он/она сможет = he/she will be able
• мы сможем = we will be able
• вы сможете = you will be able
• они смогут = they will be able

This is why you see смогу, not something built with буду.

Because after смочь (to be able to), Russian normally uses another verb in the infinitive, just like English:

• Я могу прийти. = I can come.
• Я не смогу прийти. = I won’t be able to come.

So the structure is:

subject + смочь + infinitive

Here:

• я = I
• не смогу = won’t be able
• прийти = to come

This is about verb choice and aspect.

• идти = to go on foot, to be going
• приходить = to come, to arrive regularly / repeatedly / in general
• прийти = to come, to arrive once, as a completed event

In this sentence, the speaker means to come on this specific occasion, so Russian uses прийти.

Compare:

• Я не смогу прийти сегодня вечером. = I won’t be able to come this evening.
  (one specific event)

• Я обычно не могу приходить по вечерам. = I usually can’t come in the evenings.
  (habitual/repeated)

So прийти is the natural choice here.

It means this evening or, in natural English, often simply tonight.

Word by word:

• сегодня = today
• вечером = in the evening / this evening

Together:

• сегодня вечером = this evening / tonight

In everyday translation, English often prefers tonight, even though Russian literally says today in the evening.

Because Russian often uses the instrumental case to express time when something happens, especially with parts of the day.

So:

• утро → утром = in the morning
• день → днём = in the daytime / during the day
• вечер → вечером = in the evening
• ночь → ночью = at night

That is why the sentence says сегодня вечером, not сегодня вечер.

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

• К сожалению, я не смогу прийти сегодня вечером.

You might also hear:

• Я, к сожалению, не смогу прийти сегодня вечером.
• Сегодня вечером я, к сожалению, не смогу прийти.

The basic meaning stays the same, but the emphasis changes a little.

For a learner, the original version is the best neutral pattern to copy.

Often, you can leave it out, because the verb form already shows the person:

• не смогу clearly means I won’t be able

So both are possible:

• Я не смогу прийти сегодня вечером.
• Не смогу прийти сегодня вечером.

Including я can make the sentence a little clearer, stronger, or more explicit. Omitting it can sound more natural in casual speech if the subject is already obvious from context.

It is neutral and works in many situations. It is polite enough for normal use and sounds natural in both spoken and written Russian.

You could say it to:

• a friend
• a colleague
• a host
• a teacher, depending on context

If you want to sound a bit warmer or more apologetic, you could expand it:

• К сожалению, я не смогу прийти сегодня вечером. Извините.
• К сожалению, я не смогу прийти сегодня вечером, очень жаль.

But the original sentence is already perfectly natural and polite.