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For example, thousands have tried to conquer Mount Everest.
Of those who make the attempt, only one in seven make it to the top.

In the second sentence, why the verb 'make' isn't 'makes'?

It seems that subject is 'only one',which is singular.

1. Did I misunderstand the whole sentence?




2. If there are exception for this, what else would be?

The whole paragraph is here.

Anything worth having takes time. “What we obtain too easily, we esteem too lightly,” Thomas Paine wrote. But in addition to hard work and the occasional long wait or detour through the wilderness, a key ingredient is vision. For example, thousands have tried to conquer Mount Everest. Of those who make the attempt, only one in seven make it to the top. One of the greatest factors in success versus failure is the climbers’ ability to see where they are headed. When storms blow in and obscure the top of the mountain, the climbers grow discouraged and despondent and consider retreat. But when storm air clears and the climbers see the peak again, the journey becomes easier, commitment renews, and faith is strengthened. Suddenly, getting there feels possible.

The actual subject of the second sentence is not clear because something has been omitted by ellipsis. Depending on what is omitted, we can use either make or makes.

... only one [climber] in seven makes it to the top.

... only one in seven [of those who make the attempt] make it to the top.

For the writer, the second version is the way that they intended the sentence to be understood, and for most readers the use of the plural form of the verb gives a clear indication of what's missing.

To provide a numerical example, the actual number of people that have attempted to climb Everest is more than 30,000. Only one in seven of those- but still nearly 5,000 people- succeeded. The implied subject of this sentence is those 5,000 people, so the plural form of the verb make is required.

You could also use the singular of the verb if the implied subject of the sentence is exactly one, for example

Of the days of the week, one in seven is a Sunday

There are exactly seven days in a week, so one in seven of them is one... so the verb is is singular.

Because it is plural.

"one in seven" is not a designation of one specific human who has ever succeeded in a climb (and thus being a singular).

"one in seven" is instead a fraction of "group of people" - meaning 1/7 * x, or x/7 (where x is total amount of people (note: plural) that had ever attempted to climb Mount Everest.

So, if there have been (let's say) 1000 people attempting the climb, "one in seven" actually means about 142 people, which is obviously a plural.

In other words, subject is not "only one" but instead "(only) one in seven", which you can rephrase as "(only) 14% of all people (that attempt to conquer Mount Everest)"

As to your second question: "only" before "one" does not mean there was "only one" man that have ever succeeded in climbing Mount Everest, instead it means to convey that fraction 1/7 is very small (similar as you might say "I have only $15 in my wallet" when attempting to buy something that costs more than that).

For example, thousands have tried to conquer Mount Everest. Of those
who make the attempt, [only one in seven make/makes it to the top].

The head of the bracketed noun phrase is the plural "seven", so the plural verb "make" follows the simple agreement rule.

However, the verb can be singular as well as plural, where the singular override is clearly motivated by the presence of singular "one".

Addition: Javalatte asks in comments about this example:

Of the days of the week, one in seven is / are a Sunday.

The analysis of "one in seven" is the same as your Mount Everest example. As before, the DP "one in seven" is a fused determiner-head rather than simply a determiner. The fact that these are fused-head construction doesn't seem to make much difference in principle to the verb-agreement possibilities.

The preference for the singular in this example must be motivated by the singular predicative complement, "a Sunday". The override is still in principle an optional one: that is, I wouldn't want to say that "are" is impossible here.

Compare:

Of the days of the week, one in seven are Sundays.

Here, the predicative complement is the plural "Sundays" and singular "is" would be impossible, so clearly it is the complement that motivates the verb-form: "is a Sunday" / "are Sundays.