Si queda aguacate, lo pondré sobre el pan; si no, comeré otro melocotón.

Because quedar here means to be left / to remain.

• Hay aguacate = there is avocado
• Queda aguacate = there is avocado left

So Si queda aguacate means if there is any avocado left, not just if avocado exists.

Spanish often uses food and uncountable nouns with no article when talking about an unspecified amount.

So:

• queda aguacate = there is some avocado left
• hay pan = there is bread
• compré leche = I bought milk

If you said el aguacate, you would usually mean a specific avocado or a specific batch of avocado already identified in the conversation.

In Spanish, real future conditions normally use:

• si + present indicative
• then a future, imperative, or another suitable tense in the main clause

So the pattern is:

• Si queda aguacate, lo pondré...

This is the normal equivalent of If there is avocado left, I’ll put it...

Spanish does not normally use the future after si in this kind of sentence.

Lo is a direct object pronoun, and it refers to aguacate.

Even though aguacate is a mass noun here, it is still grammatically masculine singular, so the pronoun is lo.

• el aguacate → lo
• la mantequilla → la

So:

• Lo pondré sobre el pan = I’ll put it on the bread

Because aguacate is the direct object of poner here.

You are putting something: that something is the avocado. Direct object pronouns are:

• lo / la / los / las

So:

• lo pondré = I’ll put it

Le is normally used for an indirect object, such as to him / to her / to it.

The future tense in Spanish is usually formed by adding endings to the infinitive:

• comer → comeré

But some verbs have irregular future stems. Poner is one of them:

• poner → pondr- → pondré

Other common irregular future stems are:

• tener → tendr-
• salir → saldr-
• venir → vendr-
• hacer → har-
• decir → dir-

So pondré just has to be learned as an irregular future form.

Because si no means if not.

Here the sentence means:

• if there is avocado left ... if not, I’ll eat another peach

That is exactly the two-word expression si no.

By contrast, sino as one word usually means but rather or except:

• No quiero té, sino café. = I don’t want tea, but rather coffee.

So in this sentence, si no is correct.

Because otro normally does not take un/una before it in standard Spanish.

So Spanish says:

• otro melocotón = another peach
• otra idea = another idea

Not:

• un otro melocotón

The word otro already includes the idea of another / one more.

Because Spanish often leaves subject pronouns out when the verb ending already makes the subject clear.

• pondré already means I will put
• comeré already means I will eat

So yo is unnecessary unless you want emphasis or contrast:

• Yo comeré otro melocotón, pero tú no.

In the given sentence, leaving out yo sounds natural.

Sobre means on / on top of / over.

So ponerlo sobre el pan gives the idea of placing the avocado on top of the bread.

Depending on context, Spanish speakers might also say things like:

• en el pan
• encima del pan
• untado en el pan if the idea is spreading it

But sobre el pan is perfectly understandable and emphasizes physical placement on top.

Yes, melocotón is the normal word for peach in Spain.

In many parts of Latin America, durazno is more common.

So for a learner focusing on Spanish from Spain, melocotón is exactly the form you would expect.

The semicolon separates two closely related parts of the sentence:

• Si queda aguacate, lo pondré sobre el pan
• si no, comeré otro melocotón

It works a bit like a strong comma, helping show a clear contrast between the two possibilities.

A comma would also often be possible in everyday writing, but the semicolon makes the division especially clear.