Today being Saturday, here’s a new grammar help in Eloquent English.
Today’s word is- not only… but also.
When used to introduce parallel constructions, not (only) and but (also) must each b followed by the same kind of item:
We saw not only the secretary but the chairman too (two objects).
We not only saw the chairman but spoke to him (two verbs).
Not only I but also Slyvia spoke to him (two subjects).
We spoke not only to the secretary but to the chairman (two prepositional phrases).
To word these in a way that spoils the parallel structure would be an error of style and of grammar:
We not only saw the secretary but the chairman too (one verb, one noun).
In the following examples, not only and not merely have been misplaced:
A high fat diet does not only increase the risks of CHD but also that of disorders, such as high blood pressure, certain cancers and obesity.
-Annabel Ferriman, The Observer
This should read A high fat diet increases not only the risks of CHD but also that of other disorders (two objects). Another correct version of this sentence would begin: a high fat diet not only increases the risks of CHD but also increases the risk of other disorders ( two verbs). Note that in the original sentence the construction does not only increase was incorrect.
His mistresses became matter not merely for Court gossip but had names which every housewife in the country knew.
-Peter Brent, The Edwardians
This should be rearranged to read: His mistresses not merely became matter for Court gossip but had names which every housewife in the country knew (two verbs).