Se hai due mele, ne mangio una.

Ne means of them / from that / some of it, depending on context.
Here it replaces delle mele or di quelle due mele.

So:

• Ne mangio una = I eat one of them
• more literally: Of them, I eat one

Italian uses ne very often where English uses of them, but English usually does not translate it word for word.

You can say mangio una, but it is less explicit.

• Mangio una = I eat one
• Ne mangio una = I eat one of them

Because the first part mentions due mele, Italian often uses ne to clearly refer back to that quantity. It sounds more complete and natural when you mean one of the apples.

Italian usually drops subject pronouns when they are not needed, because the verb ending already shows the subject.

• hai = you have (singular, informal)
• tu hai = you have, but with the pronoun stated for emphasis or clarity

So:

• Se hai due mele... is the normal way to say it
• Se tu hai due mele... is possible, but usually only if you want emphasis, contrast, or extra clarity

It is talking to one person, informally.

That is because hai is the second person singular form of avere.

• hai = you have (one person, informal)
• avete = you have (plural, or sometimes formal in some contexts)
• ha = he/she has or formal you have

So Se hai due mele... means If you have two apples... said to one person you address as tu.

Italian often uses the present tense to talk about a future result when the meaning is clear from context.

So:

• Se hai due mele, ne mangio una = If you have two apples, I’ll eat one
• literally, it looks like If you have two apples, I eat one

This is very common in Italian, especially in everyday speech.
You could also say Se hai due mele, ne mangerò una, but the present mangio is perfectly natural here.

Una refers to one apple.

Since mela is feminine singular, one must also be feminine:

• una mela = one apple
• ne mangio una = I eat one (of them)

If the noun were masculine, you would use uno in some cases, depending on the noun.

In Italian, numbers usually go directly before the noun without an article.

So:

• due mele = two apples
• not normally le due mele, unless you mean the two apples specifically

Compare:

• Hai due mele = You have two apples
• Hai le due mele = You have the two apples / Do you have the two apples? depending on context

In your sentence, due mele is just an indefinite quantity, so no article is needed.

Because this is a normal real condition, and after se Italian usually uses the indicative in this kind of sentence.

• Se hai due mele... = If you have two apples...

For ordinary possible or real situations, se + indicative is standard.

A common learner mistake is trying to use the conditional right after se. In standard Italian, you do not say:

• Se avresti... ❌

Instead, you use structures like:

• Se hai...
• Se avessi... for a different, more hypothetical pattern

But in your sentence, hai is exactly right.

Yes, but that would mean something different.

• Se hai due mele, ne mangio una = a real or likely condition: If you have two apples, I’ll eat one
• Se avessi due mele, ne mangerei una = a hypothetical condition: If you had two apples, I would eat one

So the original sentence sounds more direct and real.
The version with avessi / mangerei sounds more hypothetical or less immediate.

The comma is very common and helpful, because it separates the if-clause from the main clause:

• Se hai due mele, ne mangio una

In short, everyday writing, you may sometimes see it omitted, but using the comma is standard and clearer.

Yes, but adding io changes the emphasis.

• Ne mangio una = neutral, normal
• Ne mangio una io = I’ll eat one, with emphasis on I

That might suggest contrast, for example:

• Tu ne prendi una e io ne mangio una
• You take one and I eat one

So the original sentence without io is the most neutral version.

It breaks down like this:

• Se = if
• hai = you have
• due mele = two apples
• ne = of them
• mangio = I eat / I’ll eat
• una = one

So the pattern is:

Se + condition, result

Here:

• Se hai due mele = If you have two apples
• ne mangio una = I eat one of them / I’ll eat one

This is a very common Italian sentence pattern.