Je prends de la moutarde avec mes frites au lieu de la mayonnaise.

In French, prendre is very common when talking about food and drink.

Here, je prends can mean:

• I’m having
• I take
• I’ll have
• I’ll take

So in a food context, Je prends de la moutarde sounds natural for choosing what condiment you want.

It does not necessarily mean physically grabbing the mustard. It often means choosing or having it.

Because moutarde is:

1. a feminine noun, and
2. usually treated as an uncountable substance, like mustard in English.

So French uses the partitive article to mean some:

• du for masculine singular uncountable nouns
• de la for feminine singular uncountable nouns
• de l’ before a vowel sound

So:

• de la moutarde = some mustard

Since moutarde is feminine, du moutarde would be wrong.

Au lieu de is a fixed expression meaning instead of.

When it is followed by a noun, French usually keeps the article:

• au lieu de la mayonnaise
• au lieu du ketchup
• au lieu des oignons

So au lieu de la mayonnaise means instead of the mayonnaise or more naturally in English, instead of mayonnaise.

French often sounds more natural with the article here, even when English would often leave it out.

They are doing different jobs.

• de la moutarde = some mustard
  This is a partitive article, used for an unspecified amount.

• la mayonnaise after au lieu de = mayonnaise as the alternative choice
  Here French normally uses the regular article after au lieu de.

So the contrast is:

• I’m having some mustard
• instead of the/mayonnaise option

You could make the structure more parallel in a longer sentence:

• Je prends de la moutarde au lieu de prendre de la mayonnaise.

That would also be correct.

Au lieu de means instead of.

Literally, it comes from the idea of in the place of, but in normal English you should usually just think of it as instead of.

Examples:

• Je bois de l’eau au lieu de soda. = I’m drinking water instead of soda.
• Il marche au lieu de courir. = He walks instead of running.

It can be followed by:

• a noun: au lieu de la mayonnaise
• an infinitive: au lieu de prendre de la mayonnaise

Mes frites means my fries — in other words, the fries I’m eating.

French often uses a possessive when referring to something that belongs to or is associated with the speaker in that moment.

So:

• avec mes frites = with my fries

If you said les frites, it would sound more general:

• fries in general
• the fries already known from context
• a shared plate, depending on context

Both can be possible, but mes frites clearly means my portion.

Because French normally talks about fries in the plural, just like English.

• une frite = one fry
• des frites = fries

So mes frites means my fries.

The singular exists, but in normal everyday use you usually talk about the food as a plural set.

Avec simply means with.

Here it shows what the mustard goes with:

• de la moutarde avec mes frites = mustard with my fries

This is the normal way to express that kind of food pairing in French.

It does not necessarily mean the mustard is physically mixed into the fries. It just means it is the condiment or accompaniment for them.

It can mean either one. French present tense often covers both ideas.

Depending on context, Je prends de la moutarde... could mean:

• I’m having mustard... right now
• I usually have mustard...
• I’ll take mustard... when ordering

Context tells you which meaning is intended.

In this sentence, many learners would naturally understand it as a current choice, especially in a meal or restaurant context.

Because de + la does not contract in French.

Here are the main patterns:

• de + le = du
• de + les = des
• de + la = de la
• de + l’ = de l’

So:

• de le ketchup → du ketchup
• de les frites → des frites
• de la mayonnaise stays de la mayonnaise

Also, the au in au lieu de is part of the fixed expression. It is not connected to mayonnaise.

Yes, but the original order is very natural:

• Je prends de la moutarde avec mes frites au lieu de la mayonnaise.

It first says:

1. what you are choosing,
2. what you are eating it with,
3. what you are choosing it instead of.

You could also say:

• Avec mes frites, je prends de la moutarde au lieu de la mayonnaise.

That version puts extra emphasis on with my fries.

So the sentence is flexible, but the original version sounds smooth and natural.