Si tu avais pris le bon moule, la tarte aurait été plus régulière.

Because this sentence talks about an unreal past situation: something that did not happen, and the result that would have followed.

French uses this pattern for past hypotheticals:

• Si + pluperfect (plus-que-parfait)
• conditional perfect (conditionnel passé)

So:

• Si tu avais pris le bon moule = If you had used/taken the right tin/pan
• la tarte aurait été plus régulière = the tart would have been more even/neater

This is the standard French equivalent of English if you had..., would have....

Avais pris is the plus-que-parfait of prendre.

It is formed with:

• imperfect of avoir: avais
• past participle of prendre: pris

So:

• j'avais pris
• tu avais pris
• il avait pris, etc.

In this kind of sentence, after si, French uses the plus-que-parfait to describe the unreal condition in the past.

A useful pattern to remember is:

• Si + plus-que-parfait, conditionnel passé

Example:

• Si j'avais su, je serais venu. = If I had known, I would have come.

Aurait été is the conditional perfect of être.

It is formed with:

• conditional of avoir: aurait
• past participle of être: été

This tense is used to express what would have happened under a condition.

So:

• la tarte aurait été plus régulière = the tart would have been more even / would have turned out more regular

This matches English would have been very closely.

Because standard French grammar does not use the conditional directly after si in this kind of sentence.

So this is wrong in standard French:

• Si tu aurais pris...

And this is correct:

• Si tu avais pris...

A very useful rule is:

• after si, use imperfect, present, or pluperfect, depending on the meaning
• do not use the conditional there

For past unreal situations, the correct pattern is:

• Si + plus-que-parfait
• conditional perfect

Here, moule means a mold, tin, or pan used to shape baked food.

In the sentence le bon moule, it means the right baking tin/pan/mold for making the tart.

The exact English translation depends on context:

• mold is the most literal
• tin is common in British English for baking
• pan may sound more natural in some American contexts

So le bon moule means something like the right tart tin or the right pan.

Here, régulière does not mean regular in the sense of usual or frequent.

In the context of a tart, it means something like:

• even
• uniform
• well-shaped
• more symmetrical

So la tarte aurait été plus régulière suggests the tart would have had a more even shape if the right mold had been used.

Because régulière agrees with la tarte, which is a feminine singular noun.

In French, adjectives usually agree in gender and number with the noun they describe.

• masculine singular: régulier
• feminine singular: régulière

Since we have la tarte:

• la tarte aurait été plus régulière

If the noun were masculine, you would use régulier.

Because été is the past participle of être, but in aurait été, it is used with avoir as part of the conditional perfect.

In general, past participles with avoir do not agree with the subject, unless there is a preceding direct object.

Here:

• aurait is the auxiliary
• été stays unchanged

The adjective after it, however, does agree with the subject:

• la tarte aurait été plus régulière

So the agreement appears on régulière, not on été.

Literally, prendre often means to take, but in French it is also used in many contexts where English would say use, choose, or pick.

So:

• prendre le bon moule literally = to take the right mold
• but naturally = to use the right mold or choose the right pan

This is very common in French. The most natural English translation is not always the most literal one.

Yes. If you want to be formal or speak to more than one person, you can say:

• Si vous aviez pris le bon moule, la tarte aurait été plus régulière.

The grammar stays the same; only the subject changes:

• tu avais pris
• vous aviez pris

A careful pronunciation would be approximately:

Si tu avais pris le bon moule, la tarte aurait été plus régulière.

Roughly: see tyoo ah-veh pree luh bo(n) mool, lah tart oh-ray ay-tay plu ray-gyoo-lyair

A few points:

• avais sounds like ah-veh
• pris sounds like pree
• aurait sounds like oh-ray
• été sounds like ay-tay
• plus here is usually pronounced plu before an adjective
• régulière has the u sound French learners often find difficult

Yes, depending on what you want to emphasize.

For example:

• Si tu avais utilisé le bon moule, la tarte aurait été plus régulière.

This uses utilisé instead of pris, which may feel more directly like used to an English speaker.

But the original sentence is perfectly natural, and pris works well in French.

The key pattern is:

• Si + plus-que-parfait, conditionnel passé

This expresses a past situation contrary to fact.

Template:

• Si + had done X, would have done Y

French:

• Si + avais/avait/avions... + past participle, aurait/serait... + past participle

Examples:

• Si tu avais étudié, tu aurais réussi. = If you had studied, you would have succeeded.
• Si nous étions partis plus tôt, nous serions arrivés à l’heure. = If we had left earlier, we would have arrived on time.

Your sentence follows exactly this pattern.