Mon portefeuille est sur la table, et j’ai mis le reçu dedans.

Because portefeuille is a masculine noun in French.

• mon = my with a masculine singular noun
• ma = my with a feminine singular noun

So:

• mon portefeuille = my wallet

Even though a wallet is not naturally male or female, French nouns still have grammatical gender.

Portefeuille usually means wallet in everyday French.

Depending on context, it can also mean:

• briefcase
• portfolio

But in this sentence, with le reçu dedans (the receipt inside it), wallet is clearly the intended meaning.

Sur means on or on top of.

So:

• sur la table = on the table

It is used because the wallet is resting on the surface of the table. If it were inside a drawer or box, French would more likely use dans instead.

French uses articles much more often than English does.

So where English might say:

• on the table

French says:

• sur la table

In many cases, you cannot leave out the article in French. A bare noun like sur table would usually sound wrong here.

J’ai mis is the passé composé, a very common French past tense.

It is formed with:

• the auxiliary verb avoir
• plus the past participle mis from mettre

So:

• j’ai = I have
• mis = put

Together, j’ai mis means I put or I have put, depending on the context in English.

Because je becomes j’ before a vowel sound.

• je ai would sound awkward
• French contracts it to j’ai

This is called elision.

You will see this often:

• j’aime
• j’habite
• j’ai

Because mettre is an irregular verb.

Its past participle is:

• mettre → mis

Other forms include:

• je mets = I put / I am putting
• nous mettons = we put / we are putting
• j’ai mis = I put / I have put

So mis simply has to be learned as the past participle of mettre.

Because in the passé composé with avoir, the past participle usually does not agree with the direct object if the object comes after the verb.

Here we have:

• j’ai mis le reçu = I put the receipt

The direct object le reçu comes after mis, so no agreement is made.

That is why it stays:

• mis

not something different.

Le reçu means the receipt.

• reçu is a masculine noun here
• so it takes le

Be careful: reçu can also be the past participle of recevoir in other contexts, but in this sentence it is clearly a noun meaning receipt.

Dedans means inside or in it / inside it, depending on context.

In this sentence:

• j’ai mis le reçu dedans

means:

• I put the receipt inside it

Here, dedans refers back to the wallet.

It is a very natural way to avoid repeating dans le portefeuille.

French does not normally say dans lui for objects like this.

The natural choices are:

• dans le portefeuille = in the wallet
• dedans = inside it / in it

So after mentioning the wallet once, dedans is a neat and idiomatic way to refer back to it.

Yes, absolutely.

That version is also correct:

• Mon portefeuille est sur la table, et j’ai mis le reçu dans mon portefeuille.

It is just a bit more repetitive. Using dedans sounds smoother because the wallet has already been mentioned.

French word order here is fairly similar to English:

• Mon portefeuille = subject
• est = verb
• sur la table = location
• et = and
• j’ai mis = verb phrase
• le reçu = object
• dedans = location/destination idea

So the structure is very natural in French:

1. say what the wallet is doing or where it is
2. then say what you did with the receipt

Yes. Et is the normal word for and.

So:

• Mon portefeuille est sur la table, et... = My wallet is on the table, and...

A small pronunciation note: et is usually pronounced like ay.

A simple approximate pronunciation is:

• Mon portefeuille est sur la table
  mohn pohr-tuh-fuh-yay ay soor lah tahbl
• et j’ai mis le reçu dedans
  ay zhay mee luh ruh-soo duh-dahn

A few helpful notes:

• j’ai sounds like zhay
• mis sounds like mee
• reçu sounds roughly like ruh-soo
• the n in mon and dedans is nasal, so it is not pronounced like a full English n

In normal context, no. It naturally refers to mon portefeuille, because that is the thing you can put a receipt inside.

A table is not something you normally put a receipt inside, so the meaning is clear from context.

That is very common in French: the listener understands what dedans refers to based on logic and context.