Wczoraj jeden klient poprosił mnie o nową fakturę, bo na starej był błąd.

Yes. Wczoraj means yesterday, and putting it first sets the time frame right away.

Polish word order is quite flexible, so you could also say things like:

• Jeden klient poprosił mnie wczoraj o nową fakturę...
• Mnie wczoraj jeden klient poprosił o nową fakturę...

These versions are all grammatical, but they put emphasis in slightly different places.

Starting with Wczoraj is very natural if the speaker wants to begin with when it happened.

It can mean both, depending on context.

In this sentence, jeden klient often feels like:

• one client
• or a certain client / one client

It does not always strongly emphasize the number one. Sometimes Polish uses jeden a bit like English one in one customer asked me..., meaning there was this one customer.

If you just said Wczoraj klient poprosił mnie..., that would usually sound less natural unless the client was already known from context.

This is about aspect, which is very important in Polish.

• prosić = imperfective
• poprosić = perfective

Here, poprosił is used because the request is seen as a completed single event: the client asked, and that happened once.

So:

• klient mnie poprosił = the client asked me
• klient mnie prosił = the client was asking me / used to ask me / asked me repeatedly, depending on context

Since the sentence refers to one completed event yesterday, poprosił is the natural choice.

Because poprosić kogoś o coś requires the person being asked to be in the accusative case.

So:

• poprosić mnie = ask me
• poprosić cię = ask you
• poprosić go = ask him

By contrast, mi is usually the dative form of I, used in sentences like:

• Daj mi fakturę. = Give me the invoice.

So here:

• poprosił mnie o nową fakturę = he asked me for a new invoice

Because the pattern is:

• prosić / poprosić kogoś o coś = to ask someone for something

After o in this meaning, Polish uses the accusative case.

So:

• faktura → nominative
• fakturę → accusative singular

And the adjective has to match:

• nowa faktura → nominative
• nową fakturę → accusative

That is why you get o nową fakturę.

Because adjectives must agree with the noun in gender, number, and case.

Here, faktura is:

• feminine
• singular
• accusative

So the adjective nowy has to take the matching feminine accusative singular form:

• nowa faktura = a new invoice
• nową fakturę = a new invoice, as the object after o

This kind of adjective-noun agreement happens all the time in Polish.

Both are possible. The full version would be:

• bo na starej fakturze był błąd

In the sentence you have, the noun fakturze is omitted because it is easy to understand from context. Polish often does this when the adjective alone clearly refers back to a noun already mentioned.

So:

• na starej = on the old one
• more literally: on the old invoice

This is very natural in Polish.

It is the locative singular, feminine.

The phrase is really:

• na starej fakturze

When na means on in the sense of location, it usually takes the locative.

So:

• stara faktura = old invoice
• na starej fakturze = on the old invoice

Since faktura is omitted, only the adjective stays:

• na starej

And the adjective still has to remain in the same case: locative feminine singular.

Polish normally says:

• na fakturze
• na rachunku
• na liście
• na stronie

This is the standard way to say that something is written on a document, page, list, invoice, and so on.

So:

• na fakturze był błąd = there was an error on the invoice

Using w fakturze would usually sound less natural here.

Polish commonly expresses this idea as:

• na fakturze był błąd
• literally: there was an error on the invoice

That is just the natural Polish structure.

English often says:

• the invoice had an error
• there was an error on the invoice
• the invoice contained an error

Polish strongly prefers the there was an error on it type of structure here.

In Polish, new or important information often comes later in the clause. Here, the reason is revealed at the end:

• bo na starej był błąd = because there was an error on the old one

Ending with błąd sounds natural because that is the key piece of information in the explanation.

You could rearrange it, but this version sounds very normal and clear.

Because Polish often avoids pronouns when the meaning is already clear.

English says:

• because there was an error on the old one

Polish just says:

• bo na starej był błąd

The idea of one or it is built into starej, which stands for starej fakturze.

So Polish does not need a separate word like English it here.

Yes.

• bo = because
• ponieważ = because / since

In this sentence, bo is the most natural everyday choice. It sounds neutral and conversational.

You could say:

• Wczoraj jeden klient poprosił mnie o nową fakturę, ponieważ na starej był błąd.

That is correct too, but it sounds a little more formal or written.

Because in Polish, conjunctions like bo normally introduce a subordinate clause, and that clause is separated by a comma.

So:

• ..., bo na starej był błąd.

This is standard Polish punctuation.

Yes, that is grammatical.

But the nuance changes a little:

• Wczoraj jeden klient... = yesterday one client / a certain client...
• Wczoraj klient... = yesterday the client / a client, depending on context, but often it feels less complete unless the listener already knows which client you mean

So jeden klient helps introduce a new person into the story more naturally.

Yes, faktura is feminine.

You can see that from the forms around it:

• nową fakturę → feminine accusative singular
• na starej (short for na starej fakturze) → feminine locative singular

Also, dictionary form faktura ends in -a, which is very often feminine in Polish.

A very useful pattern here is:

• ktoś poprosił kogoś o coś, bo...
• someone asked someone for something because...

In your sentence:

• jeden klient = someone who asked
• mnie = the person asked
• o nową fakturę = the thing requested
• bo na starej był błąd = the reason

This is a great sentence for learning several common Polish structures at once:

• past tense
• perfective verb
• o + accusative
• na + locative
• omitted noun after an adjective
• natural word order in a short narrative sentence