Я сожалею, что не предупредил тебя заранее.

Because предупредил is perfective aspect: it treats the warning as a single completed action.

In this sentence, the speaker is sorry that they did not give the warning beforehand. Russian usually uses the perfective here because the focus is on the fact that the action never happened as a completed event.

• Я сожалею, что не предупредил тебя заранее.
  = I regret that I didn’t warn you in advance.

If you used не предупреждал, it would sound more like I wasn’t warning you / I didn’t used to warn you / I was not in the process of warning you, which does not fit this situation well.

So:

• предупредить = to warn, as a completed act
• предупреждать = to warn, in an ongoing/repeated/general sense

Because the sentence has two different time layers:

• Я сожалею = I regret / I am sorry now
• не предупредил = I didn’t warn earlier

So the regretting is happening in the present, but the thing being regretted happened in the past.

This is very natural in Russian, just as in English:

• I am sorry that I didn’t warn you earlier.

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

In что не предупредил тебя заранее, the speaker is still the person who failed to warn, so Russian does not need to repeat я.

A fuller version would be:

• Я сожалею, что я не предупредил тебя заранее.

But this sounds more explicit and often less natural unless you want special emphasis.

So Russian commonly leaves out the second я because the meaning is obvious.

Because предупредить кого? takes the accusative case.

So:

• ты = nominative, used for the subject
• тебя = accusative (and also genitive form), used here as the object

Examples:

• Ты пришёл. = You came.
• Я предупредил тебя. = I warned you.

So in this sentence, you are the person receiving the warning, not the subject, so Russian uses тебя.

Что here means that and introduces a clause explaining what the speaker regrets.

• Я сожалею, что... = I regret that...
• Мне жаль, что... = I’m sorry that...

This is a very common Russian pattern:

• Я знаю, что он дома. = I know that he is home.
• Я думаю, что это правильно. = I think that this is right.
• Я сожалею, что опоздал. = I regret that I was late.

So что connects the main statement to the subordinate clause.

They are related, but сожалею sounds more natural and appropriate here.

• сожалеть / сожалею = to regret; to be sorry about something
• жалеть / жалею can mean:
  • to feel sorry for someone
  • to pity
  • to spare
  • sometimes to regret, but in a different tone or structure

For apologetic regret, сожалею is a good formal-neutral choice.

Compare:

• Я сожалею, что не предупредил тебя заранее.
  = I regret that I didn’t warn you in advance.

• Мне жаль, что не предупредил тебя заранее.
  = I’m sorry that I didn’t warn you in advance.
  This is often even more common in everyday speech.

So сожалею is correct, but it can sound a bit more formal or deliberate than мне жаль.

Заранее means in advance, beforehand, or ahead of time.

So:

• предупредить заранее = to warn in advance

In this sentence, it shows that the speaker regrets not giving the warning early enough, before whatever happened.

Examples:

• Спасибо, что сказал заранее. = Thanks for telling me in advance.
• Надо было предупредить заранее. = You should have warned me beforehand.

So заранее adds the idea of early notice, not just warning in general.

Because in Russian, не usually goes directly before the verb it negates.

• не предупредил = did not warn

This is the normal way to negate a past-tense verb.

Examples:

• Я не знал. = I didn’t know.
• Он не пришёл. = He didn’t come.
• Мы не успели. = We didn’t manage in time.

So не предупредил is simply the standard negation: didn’t warn.

That is the past tense form of the verb in Russian.

Russian past tense is often formed from the infinitive stem plus -л, and in the singular it also shows gender:

• предупредил = masculine
• предупредила = feminine
• предупредило = neuter
• предупредили = plural

So this sentence, as written, suggests that the speaker is male.

A female speaker would say:

• Я сожалею, что не предупредила тебя заранее.

This is something English speakers often notice because English past tense does not change for gender, but Russian past tense singular does.

It is mostly neutral, but with a slightly careful or polite tone.

Reasons:

• сожалею is a bit more formal or thoughtful than very casual everyday speech
• тебя shows the speaker is addressing one person informally (ты form)

So the combination feels like: grammatically neutral, emotionally sincere, and not slangy.

A very common everyday alternative would be:

• Прости, что не предупредил тебя заранее.
  = Sorry I didn’t warn you in advance.

or

• Мне жаль, что не предупредил тебя заранее.

So your sentence is perfectly natural, just slightly more deliberate than the most casual apology.

Yes. Russian word order is flexible, though different orders can slightly change emphasis.

The original:

• Я сожалею, что не предупредил тебя заранее.

Possible variations:

• Я сожалею, что заранее не предупредил тебя.
• Что не предупредил тебя заранее, я сожалею.
  This is much less neutral and sounds literary or strongly emphatic.

The original order is the most straightforward and natural for ordinary speech and writing.

In general:

• не предупредил тебя заранее = neutral
• moving заранее earlier can emphasize the fact that it should have happened beforehand

So yes, word order can change, but the given sentence is the most standard choice.