Nous sommes sur le point d’acheter des chaussures quand Marie s’aperçoit qu’elle ne connaît plus sa pointure exacte.

Sur le point de + infinitive means to be about to do something.

So:

Nous sommes sur le point d’acheter des chaussures
= We are about to buy shoes

It suggests that the action is very close to happening, almost immediate.

French often uses être sur le point de + infinitive to express an action that is just about to happen.

It is more precise than a simple future like nous achèterons because it emphasizes the moment right before the action.

Compare:

• Nous achèterons des chaussures = We will buy shoes
• Nous sommes sur le point d’acheter des chaussures = We are just about to buy shoes

So this expression focuses on the immediacy of the situation.

Because in French, de becomes d’ before a vowel sound.

Since acheter starts with a vowel, you get:

• de acheter → d’acheter

This is called elision.

The full pattern is:

sur le point de + infinitive
but before a vowel: sur le point d’acheter

Because sur le point de is followed by an infinitive verb, just like English about to + verb.

So:

• sur le point de partir = about to leave
• sur le point de manger = about to eat
• sur le point d’acheter = about to buy

This is a fixed structure.

Des chaussures means some shoes or simply shoes in a general, indefinite sense.

Here, the sentence is talking about buying shoes, but not a specific pair already identified. So French uses the indefinite plural article des.

Compare:

• acheter des chaussures = to buy shoes / some shoes
• acheter les chaussures = to buy the shoes

If the speaker had a specific pair in mind already, les would make more sense.

This sentence uses the present tense in a kind of vivid, narrative way. French often uses the present to describe events as if they are happening right now.

So:

Nous sommes sur le point d’acheter des chaussures quand Marie s’aperçoit...

can be understood like:

We’re about to buy shoes when Marie realizes...

It makes the scene feel immediate.

In another context, especially in standard past narration, you might also see something like:

Nous étions sur le point d’acheter des chaussures quand Marie s’est aperçue...

But the version you have is perfectly natural as a lively narrative present.

S’apercevoir means to notice or to realize.

In this sentence:

Marie s’aperçoit qu’elle ne connaît plus sa pointure exacte
= Marie realizes that she no longer knows her exact shoe size

Here it is closer to realizes than just sees.

Because the verb is s’apercevoir, which is commonly used as a reflexive verb in French.

This is just how the verb is normally built in this meaning:

• s’apercevoir de quelque chose = to notice something
• s’apercevoir que... = to realize that...

So you should learn it as a unit:

s’apercevoir

It does not translate word-for-word into natural English as to perceive oneself. It simply means to notice / realize.

Que becomes qu’ before a vowel sound, just like de becomes d’.

Since elle starts with a vowel, French uses elision:

• que elle → qu’elle

So:

Marie s’aperçoit qu’elle...
= Marie realizes that she...

Ne ... plus means no longer or not anymore.

So:

elle ne connaît plus sa pointure exacte
= she no longer knows her exact shoe size

The word plus changes the meaning from simple negation to the idea that something used to be true, but is not true now.

Compare:

• elle ne connaît pas sa pointure = she does not know her shoe size
• elle ne connaît plus sa pointure = she no longer knows her shoe size

The second one suggests she knew it before, but now she doesn’t.

French often distinguishes between savoir and connaître.

A useful rule is:

• savoir = to know a fact, how to do something
• connaître = to know, be familiar with, recognize

With things like a person, place, or a piece of information treated as something familiar, French often uses connaître.

Here, connaître sa pointure means to know one’s size in the sense of being familiar with it.

So:

elle ne connaît plus sa pointure exacte
is idiomatic French.

In this sentence, pointure means shoe size.

So:

sa pointure exacte
= her exact shoe size

The word pointure is commonly used for shoe size, though in some contexts it can also refer more broadly to size.

Here, because the sentence is about buying chaussures, it clearly means shoe size.

Because pointure is a feminine noun.

So the possessive adjective must agree with the noun, not with the owner:

• sa pointure = her shoe size
• sa pointure could also mean his shoe size, if the owner were male

In French, possessive adjectives agree with the thing possessed, not with the person who possesses it.

Exacte means exact or precise.

So:

sa pointure exacte
= her exact shoe size

This suggests that Marie may know her size approximately, but not precisely enough to choose shoes confidently.

In French, most adjectives come after the noun.

So:

• une pointure exacte = an exact size
• sa pointure exacte = her exact size

Some common adjectives go before the noun, but exact is normally placed after it.

Yes. In this sentence, quand introduces the moment when something interrupts or changes the situation.

So:

Nous sommes sur le point d’acheter des chaussures quand Marie s’aperçoit...

means:

We’re about to buy shoes when Marie realizes...

Here quand works very much like English when in storytelling: one action is underway or imminent, and then another event happens.