Pago con una banconota da venti euro e tengo lo scontrino nel portafoglio.

In Italian, subject pronouns are often omitted because the verb ending already shows who the subject is.

• pago = I pay
• tengo = I keep

Both endings clearly indicate io, so adding io is usually unnecessary unless you want emphasis or contrast.

• Pago con una banconota... = neutral, normal
• Io pago con una banconota... = more emphatic, like I’m the one paying...

They are both in the present indicative.

• pago comes from pagare = to pay
• tengo comes from tenere = to keep / to hold

Here they describe a simple present action, just like English I pay and I keep.

The preposition con means with, and it is commonly used to express the means or instrument used to do something.

So:

• Pago con una banconota = I pay with a banknote

It works the same way as in English:

• scrivo con una penna = I write with a pen
• apro la porta con la chiave = I open the door with the key

Banconota is a feminine singular noun, so it takes the feminine singular indefinite article una.

• una banconota
• la banconota

This is just the normal agreement pattern:

• masculine singular: un libro
• feminine singular: una penna
• feminine singular: una banconota

In this kind of expression, da is used to show value, size, capacity, or type.

So:

• una banconota da venti euro = a twenty-euro banknote
• una bottiglia da un litro = a one-liter bottle
• un tavolo da sei persone = a table for six people

Here da venti euro describes the denomination of the banknote. Using di would sound unnatural in standard Italian for this meaning.

In Italian, euro is normally invariable: it stays euro in both singular and plural.

• un euro
• due euro
• venti euro

So venti euro is correct.

Italian uses lo instead of il before masculine singular nouns that begin with:

• s + consonant: lo scontrino, lo studente
• z: lo zaino
• ps: lo psicologo
• gn: lo gnomo
• some x or y words

Since scontrino begins with sc followed by a consonant sound cluster, it takes lo.

So:

• lo scontrino
• not il scontrino

Scontrino usually means a receipt, especially the small receipt you get from a shop, bar, supermarket, or cash register.

It is a very common everyday word in Italian. In many contexts it specifically suggests a printed till receipt rather than a formal invoice.

Nel is a contraction of:

• in + il = nel

Since portafoglio is a masculine singular noun and normally takes il, the combination becomes:

• nel portafoglio = in the wallet

Other similar contractions are:

• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

Italian uses articles very often, sometimes in places where English uses a possessive or where English is less explicit.

Here nel portafoglio literally means in the wallet, but depending on context it is naturally understood as in my wallet.

Italian often prefers:

• metto le chiavi nella borsa = I put the keys in my bag
• ho il telefono in tasca = I have my phone in my pocket

The possessive can be added if needed:

• nel mio portafoglio = in my wallet

But if ownership is obvious, Italian often leaves it out.

Because that is simply the grammatical gender of the noun:

• il portafoglio
• un portafoglio

Italian noun gender is not always predictable from meaning, so it usually has to be learned together with the article. A useful habit is to learn nouns as a unit:

• il portafoglio
• lo scontrino
• la banconota

Yes, but the meaning changes slightly.

• tengo lo scontrino nel portafoglio = I keep the receipt in my wallet
• metto lo scontrino nel portafoglio = I put the receipt in my wallet

So:

• metto focuses on the action of placing it there
• tengo focuses on the fact that I keep it there / store it there

In your sentence, tengo suggests the receipt stays in the wallet.

The given order is very natural:

• Pago con una banconota da venti euro e tengo lo scontrino nel portafoglio.

Italian word order is fairly flexible, but not completely free. You could change it for emphasis, though some versions sound more marked than neutral.

For example:

• Tengo lo scontrino nel portafoglio e pago con una banconota da venti euro.

This is grammatical, but it changes the flow and emphasis. The original sounds like a straightforward sequence of actions.

In portafoglio, the gli is pronounced like the lli in the English word million, though not exactly the same.

A rough guide:

• por-ta-FO-lyo

The stress is on fo: portafòglio.

This gli sound can be tricky for English speakers, so listening and repeating is very helpful.

Yes. In normal writing, both are possible depending on style:

• una banconota da venti euro
• una banconota da 20 euro

Writing out the number in words is often preferred in teaching materials and continuous prose, but using 20 is completely natural in many real-life contexts.