I wrote:
Here one may use the average value of the first and second squares instead of only the second square.
I doubt if I must write
Here one may use the average value of the first and second squares instead of the only second square.
Which is correct? If both are correct, what is the difference in meaning?
Ditto G-Cam, but let me add an additional comment.
As G-Cam says, "only the X", means that there are several things under discussion, but right now the one we are interested in is the X.
"The only X" says that there is only one thing that qualifies as X.
In your example, talking about the "second square", the second usage doesn't make much sense. Presumably there is only one square that is "second". I wouldn't suppose there could be 5 or 10 "second squares".
But suppose I said something about the "red square". If I said, "Consider the only red square", that would mean that while there may be many squares, only one of them is red. I am emphasizing that there is only one red square.
"Consider only the red square" says that I am calling your attention to the square that is red. The fact that I use the singular "square" implies that there is only one, but the sentence structure is not emphasizing that there is only one, but rather assumes this, and then emphasizes that this is the one we are interested in.
Both are grammatically correct but they do mean different things.
Your first example - only the second square - is drawing attention to the second square specifically (as opposed to both of the squares). The position of "only" gives the sentence a breakdown like this:
"...only [the second square]."
"The second square" refers to a specific object. This is almost certainly the correct phrasing of your intention.
The second example - the only second square - is broken down like this:
"...[the only] [second square]."
Since "the" is not immediately before "second square", this sentence is talking about "second squares" in general (rather than the specific second square you most likely are considering). It conveys a meaning that there is an object that is called a "second square" and there is only one "second square".
You should in all likelihood use
... instead of only the second square.
The other way suggests that you are overly fussy about there being just one second square.
We interpose only between determiner and noun
It is the only car we have.
when the meaning is "no other one exists/there is no other one" (adjectival only, existential quality).
When the meaning is "alone, by itself, not with anything else" (adverbial only, manner) it would be
use the second square only
use only the second square
