ambiguity?: to infinitive phrase as a purpose clause or an infinitival relative clause

Question

I think the grammar of To-infinitive is the most difficult part of learning English because it is hard for me like ESL students to know which is which. I mean, I'm, well, just wanting to classify the grammar of to-infinitive...

I learned that to infinitive phrase can be used to show the purpose or intention of subject, purpose clause.

I used a knife to cut the bread.

I submitted the recipe to win the prize money.

And I learned that to infinitive phrase can also be used as a relative clause.

There were a lot of people here to see the movie.

He is the man to wash the dishes.

And I'm lost. If it is used with two ways, I don't know what reading is preferential in some sentences.

Here are some sentences which I'm confused about.

a.We asked for a man to talk to the children

b.You need a key to unlock the door.

c.Now you can use a key to get into Google account

Are the objects in the sentences antecedents to infinitival relatives?
Or rather, are the to-infinitival clauses just purpose clauses?

As for my interpretations, Here they are.

a1. We asked for a man who was to talk to the children. [relative reading]

a2. We asked for a man (for him) to talk to the children. [purpose reading]

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b1. You need a key which is to unlock the door.

b2. You need a key in order to unlock the door.

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c1. Now You can use a key which is to get into Google account.

c2. Now You can use a key for Google account.

Which interpretations are closest in meaning to the original sentences?

I'm hoping my words get across to you.

Answer 1

I would say:

a.We asked for a man to communicate with the children

b.You need a key in order to open the door.

c.Now you can use a key to log into your Google account

I would say that is sufficient.

Answer 2

Your interpretations a1, b2, and c2 are closest.

I used a knife to cut the bread.

I used a knife in order to cut the bread : purpose clause

I submitted the recipe to win the prize money.

I submitted the recipe in order to win the prize money. : purpose clause

He is the man to wash the dishes.

He is the man that should wash the dishes. : relative infinitive clause

Answer 3

The simplest answer is that it is a purpose clause if the object that the infinitive follows does that action.

a. We asked for a man to talk to the children

A man can talk. This sentence is an example of a purpose clause.

b. You need a key to unlock the door.

A key can unlocks. This is a used as a purpose clause.

c. Now you can use a key to get into Google account.

Though this is somewhat harder to determine, the point is that the key refers to a password or a code to access the account. This is a purpose clause.

There were a lot of people here to see the movie.

In this sentence, 'here' can not see a movie. The people can see the movie. This is a relative clause.

These sentences can be interpreted differently depending on the emphasis in spoken English.

We asked for a man to talk to the children.

We specified that we want a man and not a woman to talk to the children.

We asked for a man to talk to the children.

We asked for a man to talk to and not to yell at the children.

The verbal emphasis can apply to any part of the sentence. In writing, we usually assume that the infinitive is used in a purpose clause unless it makes more sense for it to be a relative clause.

Answer 4

https://www.grammaring.com/the-to-infinitive-to-replace-a-relative-clause After this article the "to infinitive" may replace the relative clause under certain conditions! if this is correct all examples above were purpose clauses ...

the to-infinitive clause can replace a defining relative clause after ordinal numbers (the first, the second etc.), after superlatives (the best, the most beautiful etc.) and after next, last and only:

• Ethan is usually the last person to understand the joke. (Ethan is usually the last person who understands the joke.)

• His office was the next room to clean. (His office was the next room that they had to clean.)

Passive infinitives are also possible:

• His was the last composition to be marked. (His was the last composition which was marked.)“

But I think your examples are all ambiguous. The problem is that we are conditioned by teachers that there is always one solution to fit (that fits, or which? is fitting?) these examples have to demonstrate the ambiguity of sentences which can‘t be classified when they stand alone - even if the antecedent subject or the following verb is BOLD written or they are spoken with different pronunciation and varied emphasis. Only the context can make it clear. So your task is to clarify the situation by reasoning and giving an example with a relative pronoun or explaining the purpose clause. And you are arguing well.

Answer 5

The solution to your question is simple: the terms who, what or which can be replaced by a verb.

In any sentence, what you are describing as a to-infinitive is what in English is simply called the verb. Every sentence must have a verb. The infinitive form of any verb is 'to ... (action)', e.g. 'to give', 'to take', 'to put', 'to read'.

You are confusing yourself by over complicating this subject, unnecessarily. Call it 'a verb', not 'a to-infinitive', for instance.

English is a very simple language. All a sentence needs is 3 elements: a verb, a subject, and an object. So, 'the cat sat on the mat' has a verb (to sit), has someone who sits (the cat) who is the subject, and has something that is sat upon (the mat) which is the object.

As long as you have the three basic elements, the sentence is valid.

1. What is the verb? It is the word which is derived from your 'to' infinitive, i.e. 'sat' is the past tense of 'to sit'.

2. Who is doing the sitting? He is the subject.

3. What is he sitting on? That is the object.

'I used a knife to cut the bread.' The verb is 'used', the past tense of 'to use'. But there is also a second verb, in the infinitive form, 'to cut'. This second verb is easy, because it is always 'to cut'. It stays in the infinitive form, whatever else you change.

'Used' might become present tense, i.e. 'use', or perhaps even 'I am using'. In the future tense it would be 'I will use'. None of these changes to verb/subject/object modifies 'to cut', because the rules of grammar are an interplay of verb/subject, so usually don't modify other parts of the sentence. Accordingly, 'to cut' never changes.

You might be told you have to call it a 'purpose clause'.

What I say is: if you can identify the verb, the subject, and the object, then whatever remains in the sentence is the purpose.

What is the verb? To use. (Verb)
Who is doing the using? I. (Subject)
What is he using? The knife. (Object)
Why is he using it? (What is his purpose?) To cut the bread. = Purpose clause.