Paul mélange la vinaigrette, tandis que sa sœur prépare une sauce avec de la mayonnaise.

They are both in the present tense, third-person singular:

• Paul mélange = Paul mixes / is mixing
• sa sœur prépare = his/her sister prepares / is preparing

The verbs are:

• mélanger → il/elle mélange
• préparer → il/elle prépare

Because the subjects are singular (Paul, sa sœur), French uses the singular verb form.

It can mean either one, depending on context.

French often uses the simple present where English might use:

• Paul mixes the vinaigrette
• Paul is mixing the vinaigrette

Likewise:

• sa sœur prépare can mean his/her sister prepares or is preparing

So French does not need a separate form like English is mixing in many everyday cases.

Tandis que means while, and it links two actions happening at the same time.

In this sentence, it shows that:

• Paul is doing one thing
• his sister is doing another thing
• both are happening simultaneously

It can also sometimes suggest a slight contrast, a bit like whereas, depending on context.

Yes. Tandis que can mean:

• while when the focus is on two actions happening at the same time
• whereas when the focus is on contrast

In this sentence, the most natural idea is while, but there is also a mild contrast between what Paul is making and what his sister is making.

Because sœur is a feminine noun, so French uses the feminine possessive:

• sa sœur = his sister or her sister

A very important point: French possessives agree with the thing possessed, not with the owner.

So:

• sa sœur = his sister / her sister
• son frère = his brother / her brother

The word sa does not tell you whether the owner is male or female.

In French, possessive adjectives usually show the gender and number of the noun that follows, not the sex of the person who owns it.

So:

• sa sœur = his sister or her sister
• son livre = his book or her book

If French needs to make the owner clear, it usually does so through context or by adding something else, for example:

• la sœur de Paul
• sa sœur à lui in spoken emphasis

This is about the choice between the definite article and the indefinite article.

• la vinaigrette = the vinaigrette
• une sauce = a sauce

French uses la when the item is seen as specific, known, or already identified in the situation.

French uses une when introducing something as one sauce, not yet specific to the listener.

So the sentence presents:

• a specific vinaigrette being mixed
• a sauce being prepared, described as one sauce made with mayonnaise

Sometimes article choice depends less on strict logic and more on how the speaker chooses to frame the noun.

Because de la mayonnaise is a partitive article, which is often used for an unspecified amount of something.

Here it means something like:

• with some mayonnaise
• using mayonnaise

So:

• de la mayonnaise = some mayonnaise, mayonnaise as an ingredient
• la mayonnaise = the mayonnaise, a specific mayonnaise already identified

In cooking contexts, French very often uses the partitive for ingredients:

• avec du beurre
• avec de la crème
• avec du sucre

Not really, even though it looks that way word-for-word.

In this sentence, de la is functioning as a partitive article, not as a literal English of the.

So:

• de la mayonnaise usually translates as some mayonnaise or just mayonnaise
• not normally of the mayonnaise

This is one of those places where a literal translation is misleading.

French contracts:

• de + le → du
• de + les → des

But it does not contract:

• de + la → de la
• de + l’ → de l’

Since mayonnaise is feminine singular, the form is:

• de la mayonnaise

All four are feminine singular nouns:

• la vinaigrette
• la sœur
• une sauce / la sauce
• la mayonnaise

That is why you see feminine article and possessive forms such as:

• la
• une
• sa
• de la

The comma helps separate the two linked clauses:

• Paul mélange la vinaigrette
• tandis que sa sœur prépare une sauce avec de la mayonnaise

It makes the sentence easier to read and reflects a natural pause.

In French, punctuation with connectors like tandis que is often used to clarify structure, especially when two full clauses are joined.

Yes, but the meaning changes a little.

• et simply adds another action: and
• tandis que emphasizes that the two actions happen at the same time, and can also suggest a slight comparison or contrast

So et is grammatically possible in many contexts, but tandis que is more precise here.

Often yes, but there is a nuance.

• pendant que strongly focuses on simultaneous time: while
• tandis que can also do that, but it may sound a bit more formal or slightly more contrastive

So both can work, but tandis que is a good choice when you want while with a possible hint of comparison.

Sœur is pronounced roughly like sur in some English accents, but the vowel is actually a French sound that does not match English perfectly.

A few useful points:

• the œ is a single vowel sound
• the final r is the French r
• it is one syllable

If you are learning pronunciation, sœur is worth practicing because its vowel is not very intuitive for English speakers.

Avec means with, and here it introduces the ingredient used to make the sauce.

So:

• une sauce avec de la mayonnaise = a sauce made with / using mayonnaise

This is a very common pattern in French for ingredients or accompanying items:

• un café avec du lait
• une salade avec des tomates
• une sauce avec de la crème