Au café, Paul et Marie parlent doucement l'un à l'autre.

Au café sets the scene: it means something like at the café or in the café depending on context.

French often puts a place expression at the beginning of a sentence to establish where the action happens. In this sentence, Au café tells us the location before we hear what Paul and Marie are doing.

Also, au is a contraction:

• à + le = au

So:

• au café = at/to the café

Because in French, à + le must contract to au.

So you get:

• à le café → au café

This is a standard grammar rule:

• à + le = au
• à + les = aux

But there is no contraction with la:

• à la gare

And with a vowel sound:

• à l'école

The comma shows that Au café is being used as an introductory phrase.

The basic sentence is:

• Paul et Marie parlent doucement l'un à l'autre.

By moving the location to the front, French can make the setting feel more natural or more emphasized:

• Au café, Paul et Marie...

In speech, there is often a slight pause there.

Because the subject is Paul et Marie, which is they.

French verbs change depending on the subject:

• je parle
• tu parles
• il/elle parle
• nous parlons
• vous parlez
• ils/elles parlent

Since Paul et Marie is a plural subject, you need the ils/elles form: parlent.

No. In most present-tense -er verbs, the ending -ent in the ils/elles form is silent.

So parlent is pronounced like parle.

That means:

• il parle
• ils parlent

sound the same, even though they are spelled differently.

You understand the difference from the subject:

• Paul parle
• Paul et Marie parlent

Doucement is an adverb, and here it describes how they speak.

In French, adverbs of manner often come after the conjugated verb:

• Il parle vite.
• Elle chante bien.
• Ils parlent doucement.

So:

• parlent doucement = speak softly / speak quietly

This is a very normal word order in French.

Not always. Doucement can mean different things depending on context, including:

• softly
• quietly
• gently
• slowly

Here, because it is used with parler, it most naturally means:

• softly
• quietly

So it describes the tone or volume of their speaking.

L'un à l'autre means to each other or to one another.

It is a reciprocal expression: it shows that two people are doing something mutually.

So in this sentence, it tells us that:

• Paul speaks to Marie
• Marie speaks to Paul

In other words, the action goes both ways.

French often expresses reciprocity with a phrase rather than a single word.

English uses each other, but French commonly uses structures like:

• l'un à l'autre = to each other
• l'un de l'autre = of/from each other
• l'un avec l'autre = with each other

The exact preposition depends on the verb.

Since parler is parler à quelqu'un (to speak to someone), French uses:

• l'un à l'autre

Because the verb parler normally takes à before the person you speak to:

• parler à quelqu'un = to speak to someone

So when the action is reciprocal, French keeps that same preposition:

• l'un à l'autre = one to the other / to each other

The à is there because the verb requires it.

In French, when a group includes at least one male, the default grammatical gender is masculine.

Since the two people are:

• Paul = male
• Marie = female

French uses the masculine form:

• l'un à l'autre

If both people were female, you would usually write:

• l'une à l'autre

If both were male, it would also be:

• l'un à l'autre

Yes. A very natural alternative is:

• Au café, Paul et Marie se parlent doucement.

That also means that they are speaking to each other.

French often uses a reflexive/reciprocal construction with se for mutual actions:

• Ils se parlent. = They speak to each other.

The version with l'un à l'autre makes the reciprocal idea especially explicit.

You may also see both together for emphasis:

• Paul et Marie se parlent l'un à l'autre.

That is grammatically possible, though often a bit more explicit than necessary.

Because Paul et Marie are the subject of the sentence, not the object.

They are the ones doing the action:

• Paul et Marie parlent...

The indirect relationship introduced by à appears inside the reciprocal phrase:

• l'un à l'autre

If you named the person being spoken to directly, then you would use à:

• Paul parle à Marie.

But here the sentence is describing a mutual action between them, so French uses:

• l'un à l'autre

It is most naturally used for two people or things.

For more than two, French often uses:

• les uns aux autres

Example:

• Ils parlent les uns aux autres. = They speak to one another.

So in this sentence, l'un à l'autre fits well because there are exactly two people: Paul and Marie.

By itself, au café can sometimes mean to the café or at the café, depending on the verb and context.

For example:

• Je vais au café. = I’m going to the café.
• Je suis au café. = I’m at the café.

In this sentence, because it is just setting the location for parlent, the meaning is clearly at the café.

So context tells you which meaning is intended.