I came across the word подъезд [pɐdˈjest].
I am aware of the fact that there is a j added, as the e comes after the ъ. However, I wondered why it is a ъ and not a ь here. How would the word be pronounced, if it would be written like подьезд? And is there a rule that basically implies that there cannot be a ь here in this case?