When do the banks open?<br/><br/>The answer can be: I don't know when the banks are <font color="#ff0000"><b>open</b></font>. <br/><br/>Is the answer ok? <br/><br/>My doubt is why do I use infinitive after to be in this sentence? <br/><br/>Is there some rule about that situation I don't know? <br/><br/>Thanks,