Le chat veut sauter sur l'étagère, mais il ne saute que si la porte est fermée.

In French, the verb vouloir is directly followed by an infinitive without a preposition. Compare:

• Je veux partir. (I want to leave.)
• Elle veut manger. (She wants to eat.)

Adding de would be ungrammatical here.

Mais means but. It introduces a contrast between two actions:

• The cat wants to jump onto the shelf.
• In reality, it only jumps under a specific condition.

Ne … que is a restrictive construction meaning only. So:

• il ne saute que si → he only jumps if

Despite the ne, there is no negative meaning—only the restriction.

Here si introduces a condition (“only if the door is closed”).

• quand or lorsque would mean “when” in a temporal sense, not “if/condition.”

No. It’s a stative use of the past participle fermée as an adjective describing the door’s state.

• Passive voice would require an agent (e.g., la porte est fermée par Paul), but here we simply say the door is closed.

The présent de l’indicatif in French can express:

• A current action.
• A habitual or general truth.
  Here it describes the cat’s regular behaviour: “He wants to jump… but only jumps if…”

A close word-for-word:

• il → he
• ne → (marker of restriction)
• saute → jumps
• que → only
• si → if
• la porte → the door
• est → is
• fermée → closed

Putting it naturally: he only jumps if the door is closed.