One third of the mountains was higher than 1km

[keannu]

2 may not make sense, but in the structure of A(part) of B where B is plural, if a number is allocated impliciltly or explicitly to A, would you say a singular verb for A? Even in 1, we don't know the number of mountains - it could be 3 - but they just choose plural verbs.

1. I saw a lot of mountains in the region. One third of the mountains were higher than 1km...
2. (I saw three mountains in the region.) One third of the mountains was higher than 1km...

[SoothingDave]

I would use "were."

[emsr2d2]

I would use "were."

So would I, not least because it's very unlikely that there are just 3 mountains in one location.

[SoothingDave]

So would I, not least because it's very unlikely that there are just 3 mountain in one location.

Yes, they do tend to congregate.

[keannu]

This might be the question of logic, what about the following? They are unlikely to say the other way of (were).
They would say "One of the twins" instead of this, but if they say "half", does the verb also have to be "were"? I don't think so, it should be "was". Maybe the mountain case depends on common sense of numerous mountains in one location.
All I want to know finally is if A(part) of B(plural) is strict in English, but I guess not.

ex)Half of the twins was(were) so sad.

[emsr2d2]

It's irrelevant. We would not say "Half the twins ..." when talking about one of two people.

[philo2009]

The plurality of 'mountains' necessitates a plural verb here.

The number of mountains actually referred to is irrelevant.

[emsr2d2]

That's not strictly correct. It does matter how many mountains are being referred to:

- One of the mountains is taller than the others.
- Two of the mountains are shorter than the others.
- Several of the mountains are eroding.

The verb refers to the number of mountains mentioned, not to the word "mountains".

[5jj]

The verb refers to the number of mountains mentioned, not to the word "mountains".

I agree with philo on 'A third of the mountains were ...'. In the highly unlikely event that anybody would want to say this when there were only three mountains, the plural verb would be appropriate.

[philo2009]

That's not strictly correct. It does matter how many mountains are being referred to:

- One of the mountains is taller than the others.
- Two of the mountains are shorter than the others.
- Several of the mountains are eroding.

The verb refers to the number of mountains mentioned, not to the word "mountains".

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Quite so, in these cases.

In the case under discussion, however, the sentence begins, not with a pronominal form, but with a fraction, and fractions in English, in common with certain other expressions such as 'the rest', count as quantifiers. That is, they do not determine verbal number (that privilege devolving to their explicit or implicit object nouns).

One-third of the coffee WAS drunk, and the rest WAS thrown away.
One-third of the respondents WERE unhappy with the decision, but the rest WERE apathetic.
Two-thirds of the coffee WAS drunk, and the rest WAS thrown away.
Two-thirds of the respondents WERE unhappy with the decision, but the rest WERE apathetic.

etc.