Learn French: Nous décidons de restaurer une voiture ancienne.

In French, many verbs require a specific preposition before a following action. The verb décider (to decide) always uses de before an infinitive verb. You cannot just translate the English 'to' directly; you have to memorize that décider pairs with de.

Why is restaurer in the infinitive (unconjugated) form?

When a verb immediately follows a preposition like de (as it does here), that second verb must be in the infinitive form. You only conjugate the first verb in the chain (nous décidons).

Why is ancienne placed after voiture?

Ancien (and its feminine form ancienne) is an adjective that changes meaning depending on where it is placed! When placed after the noun (une voiture ancienne), it means 'old,' 'antique,' or 'vintage.' If placed before the noun (une ancienne voiture), it would mean a 'former' car (one you used to own).

Could we say une vieille voiture instead of une voiture ancienne?

Grammatically yes, but the nuance is different. Une voiture ancienne sounds prized, like a 'classic' or 'vintage' car you would want to restore. Une vieille voiture just means an 'old car,' which might imply it is worn out or rusty.