Infinitive phrase

Question

'Captain John is a person to admire'.

Can you give the way to figure out that whether it is adjectival or adverbial?

Answer 1

Captain John is a person [to admire __ ].

"To admire" is an infinitival relative clause modifying "person, where the relativised element (indicated by the gap notation '__') is object of the verb "admire".

Infinitival relatives modify nouns and nominals just like other relative clauses. Most have a modal meaning comparable with that expressed in finites by "can" and "should". Here the infinitival clause modifies "person" and is comparable to

Captain John is a person who you can/should admire.

Incidentally, I would avoid using the term 'adjectival' for the infinitival "to admire" since that implies it's an adjective, which it isn't. It's a relative clause functioning as a modifier.

Answer 2

Yes, to admire functions here as an adjectival modifying person. It is a non-finite relative clause (a person who is worthy of being admired/admirable).

As an adjective, an infinitive phrase will modify a noun in the sentence. (YourDict)

Guinlist calls it a postmodifier (coming after the noun, never before) and speaks about 3 types of noun phrases that can be formed. The first type is the case of your sentence:

Placing an infinitive (to verb) after a noun only sometimes creates a noun phrase. There are three common phrase types. One that can involve practically any noun indicates something that can or should or must be done to what the noun represents. Examples are time to spare, water to drink, nothing to see and work to do.
_{(You can add here a person to admire).}

It will be seen that the noun is like the object of the verb – it receives rather than performs the action – yet the verb is active rather than passive. In some cases, however, a passive infinitive is actually possible as well, usually to overcome a double meaning or to express a special non-passive meaning.

In another article (Nouns Combined with a “to” Verb), Guinlist differentiates to- infinitives functioning as adverbials from those that modify nouns:

To- infinitives have to be distinguished from infinitives after a noun that are not making a noun phrase with it. Compare:

(a) There was money to spare.

(b) The project needed money to be completed.

To spare in (a) makes a true noun phrase with money _{(the infinitive functions as an adjective).}. However to be completed in (b) is a separate adverbial expression indicating the purpose of all the words before, and is movable to the start of the sentence.