L’opticien sourit quand la couturière entre avec une vieille paire de lunettes.

Because French avoids a clash of two vowel sounds here. Le becomes l’ before a word that starts with a vowel or a silent h.

• le opticien → l’opticien
• la amie → l’amie

This is called elision.

In this sentence, the professions are being used as nouns meaning the optician and the seamstress, so French normally uses an article.

• l’opticien = the optician
• la couturière = the seamstress

French often does this where English might also use the. By contrast, after être when saying someone’s job, French usually drops the article:

• Il est opticien.
• Elle est couturière.

But here they are the subjects of the sentence, so the articles are needed.

The accents help show pronunciation and sometimes distinguish word forms.

In couturière:

• è usually gives an eh sound, more open than é
• è often appears in syllables like -ière

So couturière is pronounced roughly like koo-too-ree-air.

The accent is part of the correct spelling and should not be omitted in normal written French.

Sourit is the third-person singular present form of the verb sourire (to smile).

Conjugation in the present:

• je souris
• tu souris
• il/elle sourit
• nous sourions
• vous souriez
• ils/elles sourient

Since the subject is l’opticien (he/the optician), French uses sourit.

Entre is the third-person singular present form of entrer (to enter / come in).

Present tense:

• j’entre
• tu entres
• il/elle entre
• nous entrons
• vous entrez
• ils/elles entrent

The subject here is la couturière, so entre is the correct form.

French often uses the present tense for actions that are:

• happening in a general scene
• described as habitual
• told in a vivid, story-like way

So L’opticien sourit quand la couturière entre... can sound like:

• a general truth or repeated situation
• a narrative present, making the scene feel immediate

English sometimes does something similar, but not always in exactly the same way.

Here, quand means when.

It introduces the time clause:

• quand la couturière entre = when the seamstress comes in / enters

You could sometimes replace it with lorsque, which also means when, but quand is very common and natural in everyday French.

Both are possible, but they are slightly different in feel.

• une vieille paire de lunettes = an old pair of glasses
• de vieilles lunettes = old glasses

Using paire emphasizes the idea of one pair as a single object. French often talks about glasses this way.

Also notice the structure:

• une paire de lunettes

The head noun is paire (pair), which is singular, even though lunettes is plural.

Because vieille agrees with paire, not with lunettes.

The structure is:

• une vieille paire = singular feminine
• de lunettes = plural noun after de

So the adjective describes paire, not lunettes.

Compare:

• une vieille paire de lunettes = an old pair of glasses
• de vieilles lunettes = old glasses

In the second example, vieilles agrees directly with lunettes, so it becomes plural feminine.

Some common adjectives often come before the noun in French, and vieux / vieille is one of them.

So:

• une vieille paire not usually
• une paire vieille

Many adjectives of size, age, beauty, and goodness often come before the noun. A common memory aid is BAGS:

• Beauty
• Age
• Goodness
• Size

Since vieille is about age, it naturally comes before the noun.

It can mean either, depending on context.

Literally, avec means with, so the sentence only tells you that she comes in with an old pair of glasses. That could mean:

• she is wearing them
• she is carrying them
• she brings them along

In real life, because it is an optician, many readers may assume she is bringing the glasses to the optician, but the grammar itself does not force that interpretation.

In theory, French can sometimes allow ambiguity with a phrase like avec..., but in this sentence the most natural reading is that it goes with entre:

• the seamstress enters with an old pair of glasses

That is the reading most people would understand first.

If you wanted to make another meaning clearer, French would usually rewrite the sentence.

A rough English-style guide would be:

Lop-ti-syan soo-ree kan la koo-too-ree-air ahntr ah-vek yoon vyay pair duh loo-net

A few notes:

• l’opticien: the n is not strongly pronounced like English n at the end
• sourit: the final t is silent
• quand: the final d is silent
• entre: the en is nasal
• lunettes: the final s is silent

This kind of guide is only approximate, but it can help you get started.

Yes. In French, lunettes is normally used in the plural, just like glasses in English.

So you say:

• des lunettes
• une paire de lunettes

You would not normally use a singular form to mean eyeglasses in ordinary modern French.

This is why paire de lunettes works so naturally: one item, but grammatically expressed with the plural noun lunettes.