Je mets la sauce tomate sur les pâtes.

Because the verb is mettre, which means to put or to place.

In the present tense, it is conjugated like this:

• je mets
• tu mets
• il / elle / on met
• nous mettons
• vous mettez
• ils / elles mettent

So with je, the correct form is mets.

Je mets is pronounced roughly like zhuh meh.

A few useful points:

• je sounds like the s in measure
• mets sounds like meh
• The final -s in mets is normally silent

So even though it is spelled mets, you do not pronounce the s.

French often uses definite articles more than English does.

Here, la sauce tomate means the tomato sauce, but in natural English we might simply say tomato sauce.

French usually needs an article before a noun, so sauce tomate by itself would usually not sound complete in this sentence. That is why you get la sauce tomate.

For the same reason: French generally requires an article before nouns.

So:

• les pâtes = the pasta
• des pâtes = some pasta
• de la pâte = dough / paste in other contexts, so be careful

Even when English would just say pasta, French usually says les pâtes when talking about the dish in a general or definite way.

In French, pâtes is normally used in the plural when talking about pasta as food.

So:

• les pâtes = pasta

This is just the standard French way to refer to it. English uses a singular mass noun, but French uses a plural noun here.

Also, pâte in the singular usually means something different, such as dough, paste, or a mixture.

Sauce tomate is a common fixed expression in French meaning tomato sauce.

French often creates noun combinations like this, where one noun describes another. In English, we do this all the time too, as in tomato sauce or chicken soup.

You may also see expressions with de, depending on context, but sauce tomate is the normal everyday way to say tomato sauce.

Sur literally means on or onto.

So Je mets la sauce tomate sur les pâtes means that the sauce is being placed on top of the pasta.

This matches English quite closely:

• sur les pâtes = on the pasta

Other prepositions would change the meaning:

• dans les pâtes = in the pasta
• avec les pâtes = with the pasta

So sur is the natural choice if you mean you are putting the sauce on top.

Because contractions in French happen only with certain prepositions.

For example:

• à + le = au
• à + les = aux
• de + le = du
• de + les = des

But sur does not contract with les.

So you simply say:

• sur les pâtes

not a special combined form.

Because French word order here is very similar to English:

• Je mets = I put / I am putting
• la sauce tomate = the tomato sauce
• sur les pâtes = on the pasta

So the structure is:

subject + verb + direct object + prepositional phrase

That is the normal order in French for this kind of sentence.

Yes, you could, but it changes the nuance.

• Je mets la sauce tomate sur les pâtes sounds like you are talking about specific sauce and specific pasta, or about the dish in a general familiar way.
• Je mets de la sauce tomate sur des pâtes sounds more like I am putting some tomato sauce on some pasta.

So both are grammatically possible, but the sentence with la and les is very natural when talking about a meal in front of you.

It can mean either one, depending on context.

French present tense often covers both:

• I put
• I am putting

So Je mets la sauce tomate sur les pâtes could mean:

• a habitual action: I put tomato sauce on pasta
• an action happening now: I’m putting tomato sauce on the pasta

The situation tells you which meaning is intended.

• sauce is feminine singular, so you say la sauce
• pâtes is feminine plural, so you say les pâtes

That matters for articles and adjective agreement.

For example:

• la sauce chaude = the hot sauce
• les pâtes chaudes = the hot pasta

So if you later describe these nouns, the adjectives must match their gender and number.