Indosso il casco quando vado in bicicletta.

• Indossare literally means “to put on” or “to wear” clothing and accessories. It emphasizes the act of donning or having something on your body.
• Portare can also mean “to wear,” but it’s more general (“to carry” or “to bear”) and often used with glasses (portare gli occhiali) or jewelry. For helmets, jackets and most garments, Italians prefer indossare.

• In Italian, when talking about wearing clothes or gear in general, you normally use the definite article (il, la, etc.).
• Casco is a masculine singular noun, so its definite article is il.
• You would use un casco if you mean “a helmet” in the sense of “one among many” (e.g. “Ho comprato un casco nuovo”).

• Italian has a special pattern for means of transport: in + vehicle/carrier. Examples: in auto, in treno, in bicicletta.
• You can say a piedi (“on foot”) as an exception.
• Con la bicicletta is grammatically correct (“with the bicycle”), but it’s not the idiomatic way to express “riding a bike.”

Yes. Italian word order is flexible:
• Placing the time clause first adds emphasis on when the action happens.
• A comma often separates the clauses, but it’s optional. Both versions are correct.

• Both verbs are in the present indicative.
• Italians use the present for habitual actions or general truths, just like the English simple present (“I wear”).
• To stress habit, you can add adverbs: Di solito indosso il casco… or Indosso sempre il casco…

• Andare in bicicletta is the standard Italian expression for “to ride a bike.”
• Guidare is used for vehicles with a motor or steering mechanism, like cars (guidare l’auto) or motorcycles (guidare la moto).
• Using guidare with bicicletta sounds odd to a native speaker.

• Yes. Bici is the informal, shortened form of bicicletta.
• In bici is perfectly natural in everyday conversation. Example: Indosso il casco quando vado in bici.