Learn Portuguese (Portugal): A mesa debaixo do toldo está reservada para nós. - The table under the awning is booked for us.

We use estar because being booked is a temporary state resulting from an action. This is a great example of a word changing meaning depending on the verb. If you said A mesa é reservada, it would literally mean the table has a shy or private personality! For people, ser reservado means to be introverted, while estar reservado means to be set aside for a specific purpose.

Could I use sob o toldo instead of debaixo do toldo?

Yes, both mean under the awning. Debaixo de is much more common in everyday spoken Portuguese when talking about physical locations. Sob is slightly more formal or literary, but perfectly correct and widely understood.

Why is it para nós rather than por nós?

We use para because it indicates the intended recipients of the reservation (the table is meant for us to sit at). If you said está reservada por nós, you would be using the passive voice to say the table was booked by us, focusing on who made the phone call rather than who gets the table.

Why does reservada end in -a here?