Learn Hardcore French: Je mets mon casque quand je fais un tour en vélo dans le parc. - I put on my helmet when I go for a bike ride in the park.

In your sentence, je mets mon casque quand… emphasizes the action that happens each time you go for a bike ride. If you wanted to stress that you keep it on while riding, you could say:

Je porte mon casque quand je fais un tour en vélo.
(I wear my helmet when I go for a bike ride.)

Why is it mon casque and not ma casque?

Because casque is grammatically masculine in French:

un casque (a helmet)
le casque (the helmet)
mon casque (my helmet)

Possessive adjectives agree with the gender of the noun, not with the person:

mon casque (masculine noun)
ma voiture (feminine noun)
mes gants (plural)

What exactly does je fais un tour mean? Why not just je vais?

Literally:

faire un tour ≈ to do a round / to go for a spin / to go for a ride/walk.

It implies a short, non‑specific outing, often for pleasure:

Je fais un tour en vélo.
I go for a bike ride.
Je fais un tour en voiture.
I go for a drive.
Je fais un tour dans le quartier.
I go for a walk around the neighborhood.

Je vais en vélo is not idiomatic in standard French for “I go for a bike ride.”
You’d more naturally say:

Je vais au parc à vélo. – I go to the park by bike. (focus on means of transport)
Je fais un tour en vélo. – I go for a bike ride. (focus on the ride itself)

Can you explain quand je fais un tour – why the present tense and not future?

In French:

To talk about a general habit or repeated situation, you normally use the present on both sides of quand.

So:

Je mets mon casque quand je fais un tour en vélo.
= I put my helmet on when I go for a bike ride. (habit)

If you were talking about one specific future time, you’d use the future in both clauses:

Je mettrai mon casque quand je ferai un tour en vélo.
= I will put my helmet on when I go for a bike ride (later).

English often uses the present in the “when” clause and future in the main clause (I’ll put it on when I go), but French usually matches tenses: present–present, future–future.

Is en vélo correct, or should it be à vélo?

The standard, recommended form is:

✅ à vélo = by bike / on a bike

You will hear en vélo in everyday speech (especially regionally), but grammar books and teachers generally prefer:

Je fais un tour à vélo dans le parc.

Compare with other means of transport:

en voiture, en bus, en train, en avion
but
à pied, à cheval, à vélo

So if you want to be safe and “textbook‑correct,” use à vélo.

What does en vélo (or à vélo) add that just je fais un tour dans le parc wouldn’t?

Je fais un tour dans le parc.
= I go for a walk/round/trip in the park (no information about how).
Je fais un tour en vélo / à vélo dans le parc.
= I go for a bike ride in the park.

So en/à vélo specifies the means of transport (by bike).
Without it, faire un tour could be on foot, in a car, etc.

Why is it dans le parc and not au parc?

Both are possible, but they don’t highlight exactly the same thing:

dans le parc = inside the park, within its boundaries.
Focus on being physically inside the space.
au parc can mean to the park or at the park more generally, not insisting on “inside the area” as much.

In your sentence:

…en vélo dans le parc.
emphasizes riding inside the park’s paths.

If you said Je vais au parc à vélo, that’s more about going to the park by bike (the destination), not necessarily riding around inside it.

Could you say quand je fais du vélo instead of quand je fais un tour en vélo?

Yes, and it’s very natural:

Je mets mon casque quand je fais du vélo.
= I put my helmet on when I ride a bike / go cycling.

Differences in nuance:

quand je fais du vélo: general activity of biking/cycling.
quand je fais un tour en vélo: suggests a ride/outing, often shorter and more specific.

Both are fine; choose based on what you want to emphasize (the sport/activity vs the outing).

Can I change the word order and say Je mets mon casque quand dans le parc je fais un tour en vélo?

That word order sounds unnatural and heavy in modern French.

The normal options are:

Je mets mon casque quand je fais un tour en vélo dans le parc.
Quand je fais un tour en vélo dans le parc, je mets mon casque.

You can move the entire quand‑clause to the beginning or the end, but you generally don’t insert dans le parc in the middle of quand je fais un tour en vélo like in your suggested version.

Does casque only mean a bike helmet?

casque is a general word for helmet, and by context it can mean:

un casque de vélo – a bike helmet
un casque de moto – a motorcycle helmet
un casque de chantier – a hard hat (construction)
un casque audio / un casque – headphones / headset

In your sentence, because there is en vélo, mon casque is understood as mon casque de vélo.