What grammatical concept is this?

Question

I have always been confused about this pattern.

I can't be the first person to forget to pay his taxes.

That was the 11th family member to have been killed.

He can't be the person to have committed the crime.

This pattern makes me confused about when I should use the who-clause and this pattern. Why are using to-infinitive here instead of adding the who-clause?

I can't be the first person who forgets to pay his taxes.

That was the 11th family member who has been killed.

He can't be the person who has committed the crime.

My second question: Are there situations in which the use of one of them is more appropriate than the other?

My third question: What grammatical concept does this particular pattern represent?

Answer 1

First Snow in Alsace

The snow came down last night like moths
Burned on the moon; it fell till dawn,
Covered the town with simple cloths.

Absolute snow lies rumpled on
What shellbursts scattered and deranged,
Entangled railings, crevassed lawn.

As if it did not know they'd changed,
Snow smoothly clasps the roofs of homes
Fear-gutted, trustless and estranged.

The ration stacks are milky domes;
Across the ammunition pile
The snow has climbed in sparkling combs.

You think: beyond the town a mile
Or two, this snowfall fills the eyes
Of soldiers dead a little while.

Persons and persons in disguise,
Walking the new air white and fine,
Trade glances quick with shared surprise.

At children's windows, heaped, benign,
As always, winter shines the most,
And frost makes marvelous designs.

The night guard coming from his post,
Ten first-snows back in thought, walks slow
And warms him with a boyish boast:

He was the first to see the snow.

He was the first who has seen the snow.^{not OK}

With "the first", there is already a restriction, so the restrictive relative clause "who has seen the snow" is an unwanted pleonasm. But if we change it to "He was not the only one..." then either pattern is OK.