Learn Hardcore French: Je mets de la confiture sur cette tranche.

de la confiture = some jam (not all the jam, not a whole jar, just an unspecified quantity)
la confiture = the jam (all of it, or jam in general: “I like the jam / jam in general”)
une confiture = a jam (a jar/type of jam, e.g. une confiture à la fraise = a strawberry jam)

In a sentence like “I’m putting jam on this slice”, you normally want to express “some jam”, which is why French uses de la confiture.

What is the rule for using de la instead of du or de l’?

The partitive article agrees with the gender and first letter of the noun:

du
- masculine singular noun starting with a consonant
  - du pain – some bread
de la
- feminine singular noun
  - de la confiture – some jam
de l’
- singular noun (masc./fem.) starting with a vowel or silent h
  - de l’eau – some water
  - de l’huile – some oil

Since confiture is feminine, you use de la confiture.

Can you drop the de la and just say Je mets confiture…?

No. In standard French you cannot omit the article here.

Je mets de la confiture ✅
Je mets confiture ❌ (sounds foreign / incorrect)

French normally needs an article (definite, indefinite, or partitive) before a noun in these cases, whereas English can often omit “some”.

Why is the preposition sur used here? Could I say dans instead?

Sur means “on” / “on top of”, which is what you want for jam on a slice of bread.

sur cette tranche = on this slice (on the surface)

Dans means “in / inside”. It would suggest the jam is inside something (a container, a hole, etc.), so it would be wrong here:

Je mets de la confiture sur cette tranche. ✅ (on top of the slice)
Je mets de la confiture dans cette tranche. ❌ (sounds like “inside this slice”)

What does tranche mean exactly, and why doesn’t it say tranche de pain?

Tranche means “slice” (a slice cut from something).

Examples:

une tranche de pain – a slice of bread
une tranche de jambon – a slice of ham

In everyday speech, if it’s clear from context that you’re talking about a slice of bread, speakers often drop “de pain” and just say tranche:

Someone is holding a piece of bread → Je mets de la confiture sur cette tranche.

If there’s no context, the more explicit version is:

Je mets de la confiture sur cette tranche de pain.

Why is it cette tranche and not ce tranche or cet tranche?

Because tranche is a feminine noun.

The demonstrative adjectives (this/that) in French are:

ce
- masculine noun starting with a consonant
  - ce livre – this book
cet
- masculine noun starting with a vowel or mute h
  - cet homme – this man
cette
- feminine noun
  - cette tranche – this slice
ces
- plural noun (masc. or fem.)
  - ces tranches – these slices

So you must say cette tranche.

Why does French use Je mets (simple present) when English usually says “I am putting”?

French does not normally use a continuous/progressive tense (like “am putting”) in everyday speech. The simple present covers both:

Je mets… = I put / I am putting

If you really want to emphasize that the action is happening right now, you can say:

Je suis en train de mettre de la confiture sur cette tranche.
= I am in the middle of putting jam on this slice.

But in most situations, Je mets de la confiture sur cette tranche is natural and enough.

Is there a more precise verb than mettre for spreading jam on bread?

Yes, there are verbs that can sound more specific:

tartiner – to spread (butter, jam, etc.) on bread
- Je tartine de la confiture sur cette tranche.
- More idiomatic: Je tartine cette tranche avec de la confiture.
étaler – to spread out (more general)
- J’étale de la confiture sur cette tranche.

However, mettre is very common and perfectly correct; it’s neutral and easy to use.