Quand je perds la carte bancaire, je fais opposition.

It is a fixed French expression used especially in banking. Faire opposition means to put a stop on a card, to report it so it can no longer be used, or more generally to block it officially.

So in this sentence, it does not mean a literal make opposition. It is an idiomatic expression.

Examples:

• J’ai fait opposition sur ma carte. = I blocked my card / I reported my card.
• Il faut faire opposition immédiatement. = You need to report it / block it immediately.

Quand means when, but in sentences like this it can also mean whenever.

So:

• Quand je perds la carte bancaire, je fais opposition. = Whenever I lose the bank card, I block it.

A native English speaker might expect if here:

• Si je perds ma carte bancaire, je fais opposition. = If I lose my bank card, I block it.

The difference is mainly nuance:

• quand presents the action as the normal response whenever that situation happens
• si presents it more as a possibility or condition

In many real-life situations, si may sound more natural if you are talking about a hypothetical future event.

French often uses the present tense for general truths, habits, or standard reactions.

So this sentence is not talking about one specific event right now. It means something like:

• When/whenever I lose the bank card, I block it.

This is similar to English sentences like:

• When I’m tired, I go to bed early.
• If it rains, I stay inside.

If you were talking about one specific future event, French would often use the future:

• Quand je perdrai ma carte bancaire, je ferai opposition.

If you were talking about one specific past event:

• Quand j’ai perdu ma carte bancaire, j’ai fait opposition.

That is a very common learner question.

In this sentence, la carte bancaire sounds more general or generic: the bank card as a type of object.

But if you are talking about your own card, French speakers will very often say:

• Quand je perds ma carte bancaire, je fais opposition.

So:

• la carte bancaire = the bank card / bank card in a general sense
• ma carte bancaire = my bank card

For a personal real-life situation, ma carte bancaire is often more natural.

Yes, it is understandable, but depending on context, many speakers might prefer a slightly different version.

More natural possibilities include:

• Si je perds ma carte bancaire, je fais opposition.
• Quand je perds ma carte bancaire, je fais opposition.

Why?

• ma carte bancaire often sounds more natural than la carte bancaire when speaking about your own card
• si may sound more natural than quand if the idea is hypothetical rather than habitual

So the original sentence is not wrong, but it may sound a little textbook-like or general.

Because faire opposition is a fixed expression.

French often has idiomatic verb + noun combinations where the noun appears without an article, for example:

• faire attention
• avoir peur
• prendre rendez-vous

In the banking sense, faire opposition works the same way.

Saying faire une opposition would usually sound less idiomatic here. For blocking a card, the standard expression is simply:

• faire opposition

In everyday French, especially in France, carte bancaire is the most common general term for a bank card.

It can refer to the card you use to pay, whether it functions more like a debit card or a credit card.

Carte de crédit exists, but it is used less broadly in everyday French in France. It often sounds more specifically like credit card in the English sense.

So in many ordinary situations, carte bancaire is the safest and most natural choice.

Because the verb is perdre = to lose.

Its present-tense conjugation is:

• je perds
• tu perds
• il/elle perd
• nous perdons
• vous perdez
• ils/elles perdent

So je perds simply means I lose.

A learner may notice that the spelling changes across the forms. That is normal for many French verbs.

The comma separates the first clause from the main clause:

• Quand je perds la carte bancaire = subordinate time clause
• je fais opposition = main clause

In French, when a subordinate clause comes first, a comma is very common and helps readability.

It is similar to English:

• When I lose my bank card, I block it.

A simple pronunciation guide is:

Quand je perds la carte bancaire, je fais opposition.
roughly: kahn zhuh pair la kart bahn-kair, zhuh feh oh-poh-zee-syohn

A few useful points:

• quand sounds roughly like kahn
• je sounds like zhuh
• perds sounds like pair
• fais sounds like feh
• opposition ends with a nasal sound in -sion

The final written consonants in perds are not fully pronounced the way an English speaker might expect.