אחר כך הצטערתי שלא באתי, כי בסוף כולם נהנו.

אחר כך is a fixed expression meaning after that / afterward / then.

It is written as two separate words:

• אחר = after
• כך = so / thus / that way

Together, they function as one time expression. In this sentence, it sets up the sequence of events: first one thing happened, and afterward the speaker felt regret.

הצטערתי is the past tense, first person singular form of להצטער = to regret / to be sorry.

The ending -תי is the normal past-tense ending for I:

• הצטערתי = I regretted / I was sorry
• באתי = I came

So Hebrew often does not need a separate word for I, because the verb ending already tells you that.

Because שלא introduces a subordinate clause: that ... not.

Here:

• לא באתי = I didn’t come
• שלא באתי = that I didn’t come

After verbs like להצטער (to regret / be sorry), Hebrew very often uses שלא to mean that not:

• הצטערתי שלא באתי = I regretted that I didn’t come

This is more natural in Hebrew than trying to copy the English structure regretted not coming directly.

Hebrew usually uses a full finite clause here, not a gerund-like form.

So English:

• I regretted not coming

becomes in Hebrew more literally:

• I regretted that I didn’t come

That is why Hebrew uses שלא באתי rather than something equivalent to not coming.

Also, even though both verbs are in the past:

• הצטערתי = I regretted
• באתי = I came

the order of events is understood from context: first the speaker did not come, and later regretted it.

Because Hebrew verbs usually already show the subject.

In this sentence:

• הצטערתי = I regretted
• באתי = I came
• נהנו = they enjoyed / everyone enjoyed themselves

So Hebrew can leave out pronouns like אני (I) or הם/הן (they) unless the speaker wants extra emphasis.

For example:

• אני הצטערתי would sound more emphatic, like I was the one who regretted it.

Here כי means because.

So:

• כי בסוף כולם נהנו = because in the end everyone enjoyed themselves

Be aware that כי can also mean that in some contexts, especially in more formal or literary Hebrew. But in this sentence, the meaning is clearly because.

Here בסוף means in the end / eventually / as it turned out.

It does not necessarily mean the literal physical end of an event. It often gives the idea of the final outcome.

So in this sentence:

• כי בסוף כולם נהנו = because in the end everyone had fun

If you wanted to say something more literally like at the end of the party, you would usually say something fuller, such as:

• בסוף המסיבה = at the end of the party

Because כולם means everyone / all of them, and grammatically it takes a plural verb.

So:

• כולם נהנו = everyone enjoyed themselves

Even though English everyone is grammatically singular, Hebrew כולם behaves like a plural idea: all of them.

A useful extra note:

• כולם is the general/mixed or masculine form
• כולן is used for an all-female group

So if the group were all women, you might see:

• בסוף כולן נהנו

נהנו comes from ליהנות = to enjoy.

In Hebrew, this verb can be used without a direct object to mean:

• to enjoy oneself
• to have fun

So:

• כולם נהנו = everyone enjoyed themselves / everyone had fun

If you want to say what they enjoyed, Hebrew often uses מ־:

• נהנו מהמסיבה = they enjoyed the party
• literally: they enjoyed from the party

But in your sentence, no object is needed.

Yes, Hebrew word order is somewhat flexible, though different orders can slightly change the emphasis.

The given sentence:

• אחר כך הצטערתי שלא באתי, כי בסוף כולם נהנו.

This is very natural.

You could also hear:

• אחר כך הצטערתי שלא באתי כי בסוף כולם נהנו
• same meaning, just without the comma in speech

And inside the second clause, changing the order can shift the emphasis a bit:

• בסוף כולם נהנו = emphasis on the final outcome
• כולם נהנו בסוף = can sound a little more like everyone ended up enjoying themselves

Both are possible, but the original version is very natural and clear.

Sometimes, but the nuance changes.

• הצטערתי = I was sorry / I regretted
• התחרטתי = I changed my mind / I regretted a decision

In this sentence, הצטערתי is the better choice because it expresses an emotional reaction: the speaker felt sorry afterward.

התחרטתי would sound more like regretting a decision in a stronger, more deliberate sense. It is possible in some contexts, but הצטערתי fits this sentence more naturally.