Hier, j’ai croisé ta sœur au café, mais je n’ai pas osé l’appeler.

Hier means yesterday, and French often puts time expressions at the beginning of the sentence to set the scene.

So:

Hier, j’ai croisé ta sœur...

is a very natural way to say Yesterday, I ran into your sister...

The comma is just there to mark a small pause. In a short sentence like this, it is often optional, so Hier j’ai croisé ta sœur... is also possible.

In everyday French, the normal way to talk about a completed past event is the passé composé.

So:

• j’ai croisé = I met / I ran into

It is built with:

• avoir in the present: j’ai
• plus the past participle: croisé

The literary past form je croisai exists, but it is mainly found in formal writing, literature, and storytelling, not in normal conversation.

Yes, croiser literally relates to crossing, but when you say croiser quelqu’un, it often means:

• to run into someone
• to come across someone
• to meet someone by chance

So here j’ai croisé ta sœur does not mean you physically crossed her. It means you happened to see or encounter her.

French possessive adjectives agree with the thing possessed, not with the person you are talking to.

So:

• ta sœur because sœur is feminine singular
• even if the listener is male

Compare:

• ton frère = your brother
• ta sœur = your sister

So ta does not mean the person addressed is female. It only matches sœur.

Sœur is pronounced roughly like sur said with rounded lips, though there is no exact English equivalent.

The spelling œ is a common French letter combination called a ligature. You will also see it in words like:

• cœur = heart
• œuf = egg

The final r in sœur is pronounced, unlike many final consonants in French.

Because à + le contracts to au in French.

So:

• à le café → au café

This is a fixed rule.

Here au café means at the café. With this sentence, it gives the location where the speaker met the sister.

Other common contractions are:

• à + les → aux
• de + le → du
• de + les → des

In the passé composé, the negative usually goes around the conjugated auxiliary verb.

So:

• j’ai osé = I dared
• je n’ai pas osé = I didn’t dare

Pattern:

• ne + auxiliary + pas + past participle

So French says:

• je n’ai pas osé
• je n’ai pas vu
• je n’ai pas compris

not:

• je n’ai osé pas

Also, ne becomes n’ before a vowel sound, which is why you get n’ai.

Because oser is one of the French verbs that can be followed directly by an infinitive.

So:

• oser faire quelque chose = to dare to do something

Examples:

• j’ose parler = I dare speak
• elle n’a pas osé demander = she didn’t dare ask

French does not need a preposition here. English uses to, but French just uses the infinitive directly.

L’ stands for la, referring back to ta sœur.

So:

• appeler ta sœur → l’appeler

French often replaces a repeated noun with a direct object pronoun.

It becomes l’ instead of la because appeler begins with a vowel:

• la appeler is impossible
• l’appeler is correct

So this part means to call her.

Because the pronoun belongs to the infinitive appeler.

In French, when you have a conjugated verb plus an infinitive, the object pronoun usually goes right before the infinitive it goes with:

• je veux l’appeler = I want to call her
• je vais le voir = I am going to see him
• je n’ai pas osé l’appeler = I didn’t dare call her

So l’ is the object of appeler, not of oser.

Not necessarily. Appeler can mean:

• to call
• to call out to
• to phone

In this sentence, because the speaker saw her at the café, the most natural meaning is probably to call out to her or to speak to her.

So the idea is likely: I saw your sister, but I didn’t dare call out to her / get her attention.

Both are possible in French, but they give slightly different shades of meaning.

• je n’ai pas osé = I didn’t dare, in that particular moment
• je n’osais pas = I wasn’t daring to / I didn’t feel able to, more as an ongoing state or hesitation

In this sentence, the speaker is talking about one completed event yesterday, so passé composé fits very well.

It presents the hesitation as part of that specific situation: I ran into her, but at that moment I didn’t dare call her.

Because with avoir, the past participle usually does not agree with the direct object if that object comes after the verb.

So:

• j’ai croisé ta sœur
• not j’ai croisée ta sœur

Here ta sœur comes after croisé, so there is no agreement.

Likewise in je n’ai pas osé l’appeler, osé stays unchanged here.

This is a very common point in French grammar: with avoir, no agreement unless there is a preceding direct object in the right grammatical situation.