Je mets de la confiture sur cette tranche.

Mets is the first person singular (je) form of the verb mettre in the present tense.

• Mettre most often means “to put” or “to put on”.
• So je mets = “I put / I am putting.”

In this sentence, Je mets de la confiture… = “I am putting jam…”

Mettre is irregular. Here is its present tense:

• je mets – I put / am putting
• tu mets – you put / are putting (singular, informal)
• il / elle / on met – he / she / one puts / is putting
• nous mettons – we put / are putting
• vous mettez – you put / are putting (plural or formal)
• ils / elles mettent – they put / are putting

Note that the stem changes (met- / mett-) and endings are not regular -re endings.

De la confiture is a partitive article and usually translates as “some jam” (an unspecified amount).

• 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.

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.

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”.

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”)

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.

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.

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.

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.