Learn English – Comma problem in a long sentence (not only ~ but also)

If a clause makes sense on its own, then it is an independent clause (an independent clause will always have a subject and a verb). You do have to put a comma before "but" if it precedes an independent clause.

For example:

Tom not only forgot his wallet, but he also forgot his phone

In the above example, "he also forgot his phone" makes sense on its own, thus it is an independent clause and must be preceded by a comma.

Now take this example:

Tom forgot not only his wallet but also his phone

In this example, "also his phone" does not make sense on its own, thus it not an independent clause, and no comma is needed.

For your last sentence, you are correct to omit the comma, because "but also kind" doesn't make sense on its own, and is not an independent clause:

Tom is not only smart but also kind

However, you could change the sentence to make it into an independent clause, in which case you would need a comma:

Tom is not only smart, but he is also kind

The parallelism rule doesn't say that everything that satisfies the rule is correct and nice: it just says that everything that does not satisfy the rule is wrong.

You can divide a sentence up in lots of different ways so that each clause contains the same grammatical components:

1) Tom not only has friends in America but also has friends in Canada.

2) Tom has friends not only in America but also in Canada.

3) Tom has friends in not only America but Canada.

All of these satisfy the rule: not all of them are equally nice. For me, the second example sounds nicest so that's the one that I would use.