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Is using "whether ... than ..." in the following grammatically correct? "This applies whether you have a closer connection to a foreign country than the United States during 2018."

The phrase comes from the IRS publication 519 (2018), Chapter 1, Section "Dual-Status Aliens", Subsection "Last Year of Residency", Paragraph "Residency during the next year", with my bold.

Residency during the next year. If you are a U.S. resident during any part of 2019 and you are a resident during any part of 2018, you will be treated as a resident through the end of 2018. This applies whether you have a closer connection to a foreign country than the United States during 2018, and whether you are a resident under the substantial presence test or green card test.

(I should add that I am obviously not looking for legal advice: in any case this paragraph does not apply to me.)

The phrase is saying "whether you have a closer connection to a foreign country than you do to the United States". I see no cohesion between whether and than here.

In the sentence

whether

is used as a function word to indicate an indirect question involving stated or implied alternatives
(https://www.merriam-webster.com/dictionary/whether).

Than

is not connected with 'whether' but with the comparative form
closer .

The sentence in question is perfectly grammatical. It follows the following pattern:

X is applicable whether a comparison is true (you have a closer connection to a foreign country than the United States during 2018).

This said, there's no relation between whether and than.

It's not strange. It's correct. Let's make it easier to digest by putting the subordinate clause first:

"Whether (or not) you have a closer connection to a foreign country
than the United States during 2018, this applies to you."

I added "or not" in parentheses because it is meant but is left implied, which is often the case in English and is the case in this sentence. Maybe this is what's throwing you off, like by you mistakenly thinking the "than" is providing the "or" alternative for "whether" that's been omitted, "than" and "or" both being conjunctions.

Here's a simplified version of the sentence:

"Whether or not you have a closer friend than Uncle Sam, this applies."

To be clear, the above sentence doesn't mean the same thing as your sentence. It is merely a similar sentence that doesn't use so many words so it becomes clearer what the structure is and what's being said.

It's ungrammatical, because "other" has been mistakenly omitted. It should have been "... a closer connection to a foreign country other than the United States". The "whether" has nothing to do with it.

It might not be ungrammatical in a dialect of English other than mine (contemporary Midwestern American), but without the "other", it just doesn't work for me, at all.