The phrase "not only" is a perfectly acceptable and frequently used part of speech in written English
You would typically use it when emphasizing a particular point or action that may seem unexpected or unusual. For example: "My college experience not only helped me develop my academic skills, but also helped me become a more confident person."
Pitched as a novel for young adults, it not only proves positive for the future of writing but also for the future of reading.
If you can let the transfixing music take you to this metaphysical sphere, you adjust to Wagner time, and the opera not only proves riveting but also seems the length it must be.
He knows every word as he sings along to their hits (hitting all the notes as he goes), which not only proves his bona fides as a music enthusiast, but also makes the singers feel at ease.
But if the election not only proves inconclusive but also leads to weeks of political haggling rather than decisive action, investors will inevitably take a long, hard look at Britain.
It is the Negro, of course, who is presumed to have become equal — an achievement that not only proves the comforting fact that perseverance has no color but also overwhelmingly corroborates the white man's sense of his own value.
The Xs are independent and have the same distribution, E[n−1 X1 +⋯+ Xn)] = μ and Var[ X1 +⋯+ Xn)/n] = Var X1)/n, so that This not only proves equation (11), but it also says quantitatively how large n should be in order that the empirical average, n−1 X1 +⋯+ Xn), approximate its expectation to any required degree of precision.
Yet he has not only proved inconveniently speedy and conclusive.
A Jean presidency could not only prove unwise, it could prove disastrous.
Data mining has not only proved to be unsuccessful here, but it is actually harmful.
They say the case demonstrates that DNA can not only prove innocence, but also pinpoint culprits.
The Kremlin has not only proved incapable of ignoring Limonov; it has also adopted his tactics.
