# only you and your sons

Here "you" and "your" are singular and refer to Aaron. Other occurrences of "you" and "your" are plural and refer to Aaron and his sons. 

# exercise the priesthood

"do the work of priests"

# everything inside the curtain

"everything in the room behind the curtain" 

# Any foreigner who approaches must be put to death

"Any foreigner who approaches must die" or "You must put to death any foreigner who approaches" 

# who approaches

"who approaches the sacred things"