What's better? it's not accidentally there or it's not there accidentally?

Question

What's better?

a) It's not accidentally there.

or

b) It's not there accidentally.

The context: She saw the spark in my eyes, and I told her that it's not accidentally there.

Answer 1

From Cambridge Dictionaries Online...

We can put adverbs and adverb phrases at the front, in the middle or at the end of a clause.
...
Different types of adverbs go in different places.

Adverbs of manner
usually go in end position
She ate quickly.

sometimes go in mid position
(if the adverb is not the most important part of the clause or if the object is very long)
She quickly ate her dinner and ran out.

I'd say accidentally is an "adverb of manner". Though to be honest, looking at...

1: She belched accidentally.
2: She accidentally belched.
3: She belched accidentally and brought shame on the family.
4: She accidentally belched and brought shame on the family.

...I think they're all perfectly normal sequences. There's a potential nuanced difference in that 1 & 3 could imply she sometimes belches deliberately, but it wouldn't always be intended or understood.

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Turning to OP's specific example, as @The Photon comments, alternative (and probably more likely) phrasing is It's not there by accident. But (finally addressing the question as asked! :) I think both OP's alternatives are "credible" - but in practice the vast majority of native speakers would honour the default principle adverb of manner = end position (not that native speakers think such things; they just say what feels "natural"). Thus, the most likely (not "best") version is...

It's not there accidentally.

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Note that if we take a closely-related usage...

5: I didn't become a father accidentally.
6: I didn't accidentally become a father.

With no other context to go on, the default adverb of manner at the end principle leads to the natural interpretation of 5 as I deliberately became a father. But unless context (or delivery: stressing the adverb) suggests otherwise, 6 will be taken to imply I didn't become a father at all.

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I go to this level of detail because I think although there might be idiomatic exceptions, most adverbs of manner really do tend to go in the "end position", and by implication they tend to go after the relevant verb even if there's more text following. So it's a principle (not "rule") worth knowing, if you don't have the benefit of being a native speaker (we "accidentally" give the impression of knowing the principle).

Two final examples. One year after you got married, if your wife asks you one of...

7: Did you accidentally forget our wedding anniversary?
8: Did you forget our wedding anniversary accidentally?

...you're more likely to see a second anniversary if you hear 7 and your wife is a native Anglophone. By the default principle above, the adverb isn't very important, so it's thrown in before the main verb forget. In 8, the implication (as with 5 above) is that you did forget, and your wife is so angry and/or suspicious she's prepared to imply you might have done this on purpose (not a good sign!).