I was told when the statement starts by "not only" this one must be produced like an interrogative sentence, for example:
here we are the original statement: "Not only airports are part of the target customer group, but also other large infrastructures".
it was changed into:
"Not only are airports part of the customer group target , but also other large infrastructures".
however, if we want to use " not only" after the subjet of the statement, we can do it this way:
airports are not only part of the customer group target , but also other large infrastructures".
finally, we can change "but also" by "but...as well". Example:
airports are not only part of the customer group target , but other large infrastructures as well".