Question mark when ending sentence with "I wonder"?
I know that when a sentence begins with "I wonder" followed by a question, it is typically ended with a period.
I wonder why that is.
However, when "I wonder" is moved to the end of the sentence, does the same rule apply?
Why is that, I wonder?
It seems harder to deny the writer a question mark in this case if one is felt.
Solution 1:
Both are correct English.
The first sentence has an embedded question object complement of a mental perception verb, and, as described here, sentences like these are often intoned as questions, because they're intended as requests for information. When a writer wishes their sentence to sound like a question in the mind's ear of the reader, they use a question mark; otherwise, not. This is very ordinary.
The second sentence has undergone a Dislocation, wherein the embedded question is moved to the front of the sentence, presenting a question form and signalling a bald request for information. And therefore, it is almost always punctuated with a question mark in writing; this is also very ordinary.
Solution 2:
I wonder why that is. Is a question asked in a manner not expecting any answer in response. It is in it's own way telling the answer.
Whereas Why is that, I wonder? is asked to have a answer in response, that's why we use a question mark over here.
Solution 3:
A quick survey of Google Books reveals that in both forms are commonly written with, and without a question mark.
Without a question mark, both forms become a statement of fact.
I wonder why that is.
Why is that, I wonder.
You can imagine them spoken with a downward inflection on the last syllable.
With a question mark, they are questions, perhaps rhetorical, but you can imagine them being spoken with an upward inflection, as if inviting an answer.
I wonder why that is?
Why is that, I wonder?
Solution 4:
If you phrased your sentence,
I wonder, why is that?
it would require a question mark as well. The distinction is not where "I wonder" occurs, but the subject-verb inversion in "why is that"/"why that is". Subject-verb inversion is the grammatical way to form questions using the "wh" words which, why, how, who, what, and where. Grammatically, your first sentence is a statement.