I would go with "it can be easily obtained".
"It can easily be obtained" sounds fine, too, but "it easily can be obtained" doesn't. The complete Google stats look as follows:
Searching the British National Corpus returns these results:
- it can easily be — 40
- it can be easily — 20
- it easily can be — 0
English originally had "further" as the comparative form of "fore" and "farrer" as the comparative of "far." "Farther" came later, presumably as a back-formation, and has been interchangeable with "further" throughout most of the history of the written language. The differentiation between "further" and "farther" seems to have been invented ca. 1900 by grammarians. It's not a distinction that has ever been widely accepted, and it's not worth fighting to maintain it because it doesn't enrich the language. Because it has never been a natural part of the language, there is not widespread agreement on what criterion should be used to distinguish them: figurative versus literal, or addition versus distance. There is no clear evidence that the best writers have systematically observed the distinction. "Further" can be used literally:
She said the morning was so beautiful that she had walked further than she intended [...] -- Dickens, Martin Chuzzlewit, 1844
"Farther" can be used figuratively:
If you can bear your load no farther, say so. Is it manly to bring new sufferings on us all by irritating those whom we cannot resist? -- H. Martineau, The Charmed Sea, 1834
To many people's ears, including mine, the figurative use of "farther" sometimes sounds a little off ("I could use some farther income."), but that doesn't mean it's incorrect. Literal "further" may be more common in British English than in American English.
There is no value in trying to fight for the distinction, since it doesn't help us to make shades of meaning; the meaning is either literal or figurative based on the context, and this distinction can't be blurred or confused by which form of the word we choose.
Even relative prescriptivists such as Fowler (Fowler's Modern English Usage, 2e) don't advocate for maintaining the difference between the words, and the OED doesn't find much historical support for it.
The following points may be helpful for nonnative speakers:
- There is a transitive verb "further," but no transitive verb "farther." There are related forms like "furthering" and "furtherance," but no such forms based on "farther."
- We have "furthermore" but not "farthermore."
- The usage that is most likely to sound wrong to 21st-century speakers is "farther" in the figurative sense.
For all of these reasons, nonnative speakers are safer saying "further" when in doubt. Substituting "further" for "farther" is never incorrect, and many native speakers use "further" exclusively. Fowler speculates that "further" is eventually going to displace "farther" completely.
Best Answer
There is a complication with 'further'.
One usage is the 'furthermore' sense
[AHD]
The question with ambiguous cases is which sense is intended.
With
one possible reading is the pragmatically marked
Whereas with
there is obviously a reference to an increased extension.
So with your second sentence, further must be modifying extended, while with your first sentence, it is unclear rather further is a verb-modifier or a sentence-modifier.