Păstrez chitanța în portofel, lângă card.

Because chitanța is the definite form of the noun chitanță. Romanian usually adds the definite article to the end of the noun instead of putting a separate word like the in front.

So:

• chitanță = a receipt
• chitanța = the receipt

Here the speaker means a specific receipt, so chitanța is the natural choice.

Yes, but it changes the meaning. O chitanță means a receipt, so it sounds nonspecific. Compare:

• Păstrez chitanța... = I keep the receipt...
• Păstrez o chitanță... = I keep a receipt...

Also, Păstrez chitanță... without any article would normally sound incomplete or unnatural in standard Romanian.

Romanian uses pe mainly with direct objects that are people, pronouns, or sometimes highly specific animate beings. Since chitanța is an inanimate object, pe is not used here.

So:

• Văd fata = I see the girl
• O văd pe fata aceea = I see that girl

But:

• Păstrez chitanța = correct
• Păstrez pe chitanța = not correct here

Romanian does not always use the article in the same places English does. After a preposition, an unarticulated noun can be perfectly natural when the location is simply being stated and the context makes it clear enough.

So în portofel can naturally mean something like in the wallet or in my wallet, depending on context.

If you want to be more explicit, you can say:

• în portofelul meu = in my wallet
• în portofelul acesta = in this wallet

For a similar reason: Romanian often leaves the noun without the definite article after a preposition unless the speaker wants to make the reference especially explicit.

So:

• lângă card = next to a card / next to the card, depending on context
• lângă cardul meu = next to my card
• lângă cardul bancar = next to the bank card

Also, card is a normal modern Romanian word, borrowed from English.

They are both prepositions:

• în = in / inside
• lângă = next to / beside

In this sentence, they take the accusative. In practice, that does not usually change the visible form of nouns like portofel and card, so they look the same as they do in the dictionary.

Also, chitanța is the direct object of păstrez, so it is accusative too, even though its form is the same as the nominative definite singular.

Not strictly. The sentence works with or without the comma.

• Păstrez chitanța în portofel lângă card.
• Păstrez chitanța în portofel, lângă card.

With the comma, lângă card feels a bit more like an added detail or afterthought. Without it, the whole location phrase feels more tightly connected. In everyday writing, many people would leave the comma out.

Yes. Romanian word order is fairly flexible, though the original sentence is the most neutral.

Neutral:

• Păstrez chitanța în portofel, lângă card.

With emphasis on chitanța:

• Chitanța o păstrez în portofel, lângă card.

With emphasis on location:

• În portofel, lângă card, păstrez chitanța.

Notice that when the direct object is moved to the front, Romanian often repeats it with a clitic, as in Chitanța o păstrez...

Because Romanian often leaves possession understood when it is obvious from context. If I say în portofel, listeners will often naturally understand in my wallet unless the context suggests otherwise.

But you can absolutely add the possessive if you want to be explicit:

• în portofelul meu
• lângă cardul meu

Adding meu makes the reference more specific, but it is not always necessary.

The main pronunciation points are these:

• ch in chitanța is a hard k sound here, so it starts like ki-, not like English chair
• ă in păstrez and lângă is like the a in about
• â and î are the same Romanian vowel; there is no exact English equivalent
• ț is ts, like in cats

A rough English-style approximation is:

puh-STREZ kee-TAN-tsa uhn por-to-FEL, LUN-guh kard

That is only approximate, especially for â/î, which is a specifically Romanian sound.