Ela usa capacete quando anda de trotinete.

In Portuguese, usar means “to wear” in the sense of habit or state: Ela usa capacete = “She wears a helmet.”

• Vestir is mainly “to put on/wear clothing (garments),” not accessories/gear like a helmet.
• Pôr means “to put (on),” so it’s for the action of putting it on: Ela põe o capacete (“She puts the helmet on”).
  For a general habit, usar is the natural choice.

With clothing/gear in a general habitual statement, Portuguese often omits the article: usa capacete, usa óculos, usa uniforme.

• usa um capacete is possible but sounds like “wears a helmet (one helmet)”—you’d use it when introducing one as an indefinite object.
• usa o capacete points to a specific helmet already known in context.

The preposition de expresses the means of transport: andar de trotinete, ir de carro, ir de comboio, andar de bicicleta.
Using na (“on/in the”) would refer to a specific vehicle and is not the standard way to express transport mode. Compare:

• Vou de comboio = I go by train (mode)
• Vou no comboio das 8 = I go on the 8 o’clock train (a specific train)

Andar alone often means “to walk” or “to move around.” With de + vehicle, it means “to ride/go by”:

• andar = to walk
• andar de trotinete = to ride/go by scooter

It’s the simple present describing a habit.
For a future, Portuguese uses the future subjunctive after quando:

• Habit: Ela usa capacete quando anda de trotinete.
• Future: Quando andar de trotinete, ela vai usar/usará capacete.

Yes: Quando anda de trotinete, ela usa capacete.
Use a comma when the dependent clause comes first; no comma when it comes after:

• Ela usa capacete quando anda de trotinete.

Approximate EP pronunciation:

• Ela ≈ “EH-luh”
• usa ≈ “OO-zuh” (z as in “zoo”)
• capacete ≈ “kah-pah-SEH-t(uh)” (final e very weak)
• quando ≈ “KWAHN-doo” (ã is nasal)
• anda ≈ “UN-duh” (a nasal at the start)
• de ≈ very short “d(uh)”
• trotinete ≈ “tro-tee-NEH-t(uh)”
  Native EP reduces many final e’s to a very weak sound.

Yes, with a nuance:

• andar de trotinete emphasizes the activity/ability or habitual mode.
• ir/vai de trotinete emphasizes going somewhere by scooter on that occasion.

In a habitual sentence, quando can mean “whenever.” You can also use sempre que for extra clarity:

• Ela usa capacete quando anda de trotinete.
• Ela usa capacete sempre que anda de trotinete.

Yes, andar is a regular ‑ar verb (present):

• eu ando, tu andas, ele/ela/você anda
  Examples: Eu ando de trotinete. / Tu andas de trotinete. / Ela anda de trotinete.