J’ai oublié mon portefeuille, si bien que je ne peux pas payer en espèces.

J’ai oublié is in the passé composé, a very common French past tense. It means I forgot or I have forgotten.

• j’oublie = I forget / I am forgetting (present tense)
• j’ai oublié = I forgot / I have forgotten (past)

The speaker is talking about something that already happened: they forgot their wallet, and now that past action has a present consequence.

J’ai is a contraction of je ai, but French does not keep those two vowels next to each other, so it becomes j’ai.

It is made of:

• je / j’ = I
• ai = have (from avoir)

So j’ai oublié literally looks like I have forgotten.

In j’ai oublié, the verb uses avoir as the auxiliary verb. With avoir, the past participle usually does not agree with the subject.

So:

• J’ai oublié = said by a man or a woman
• no change is needed here

You would only see agreement in some cases, such as when a direct object comes before the verb, but that is not happening in this sentence.

Mon means my, and it must agree with the noun possessed, not with the owner.

Since portefeuille is:

• singular
• masculine

you use:

• mon portefeuille = my wallet

Compare:

• mon livre = my book
• ma voiture = my car
• mes clés = my keys

Si bien que is a linking expression meaning so, so that, or with the result that.

It connects the first idea to its consequence:

• J’ai oublié mon portefeuille = I forgot my wallet
• si bien que je ne peux pas payer en espèces = so I can’t pay in cash

It is slightly more formal or polished than simply saying donc in some contexts.

Yes, often you could say:

J’ai oublié mon portefeuille, donc je ne peux pas payer en espèces.

That would sound natural too. The difference is mostly in tone and structure:

• donc = very common, straightforward so / therefore
• si bien que = emphasizes the consequence a bit more, often sounding a little more formal or literary

Both are understandable and correct in many everyday situations.

This is the normal French negation pattern:

• ne ... pas = not

So:

• je peux = I can
• je ne peux pas = I cannot / I can’t

The verb here is pouvoir (to be able to / can), and peux is the je form in the present tense.

In spoken informal French, many speakers drop ne:

• written/formal: je ne peux pas
• casual spoken: je peux pas

But in standard written French, you should keep ne. Since your sentence is written standard French, je ne peux pas is the correct form.

After pouvoir (can / to be able to), French uses an infinitive:

• je peux payer = I can pay
• je peux venir = I can come
• je peux partir = I can leave

So in the negative:

• je ne peux pas payer = I can’t pay

This is similar to English, where can is followed by a base verb.

En espèces means in cash.

So:

• payer en espèces = to pay in cash

This is a fixed expression. French does not usually say it the same way as English word-for-word. It is best learned as a chunk:

• payer en espèces = pay in cash
• payer par carte = pay by card

Not in this sentence. Although espèce can mean species in other contexts, in the expression en espèces, it means in cash.

So this is an idiomatic expression, and the meaning depends on the whole phrase, not just the single word.

The sentence describes:

1. a past action: I forgot my wallet
2. a present result: I can’t pay in cash now

That is why French uses:

• past tense for j’ai oublié
• present tense for je ne peux pas

This is very natural, because the forgetting happened earlier, but the inability to pay is happening now.

In everyday speech, many people might say:

J’ai oublié mon portefeuille, donc je peux pas payer en espèces.

Differences:

• donc instead of si bien que
• dropping ne in speech: je peux pas

But the original sentence is completely correct and a bit more polished in style.

Yes, portefeuille can be tricky for English speakers.

A rough guide:

• por-te-feuille
• the last part feuille sounds somewhat like fuhy or fœy to English ears, though not exactly

Important points:

• the r is French
• eu and euille do not sound like normal English vowels
• the final lle is not pronounced like a strong English l

It is a good word to listen to several times rather than rely only on spelling.

It is mostly neutral to slightly formal.

Why?

• J’ai oublié mon portefeuille is neutral everyday French.
• je ne peux pas payer en espèces is also neutral.
• si bien que makes the sentence sound a little more formal or carefully structured than everyday casual conversation.

So it is perfectly correct, but in casual speech many people would choose a simpler connector like donc.