# After they had passed by the first guard and the second

"The first and second guards did not see them as they passed by, and then"

# and the second

"and the second guard"

# they came to the iron gate that led into the city

"Peter and the angel arrived at the iron gate that went from the prison to the city"

# it opened for them by itself

"the gate swung open for them" or "the gate opened itself for them" 

# went down a street

"walked along a street"

# left him right away

"left Peter suddenly" or "suddenly disappeared"