I am not able to understand why the infinitive must be used after "I am qualified to".

For example

I am qualified to teach.

Does not to play the role of preposition in this sentence?

If the question above is true, should not a gerund be used?

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    Note that the "to" in "I am qualified to" is properly part of the infinitive (the infinitive in English is two words: "to teach"). You've baked the answer into the question. – Dan Bron Sep 15 '14 at 16:42
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    If you wanted to use a gerund, you'd have to use "for" as the preposition: "I am qualified for teaching". – Dan Bron Sep 15 '14 at 16:51
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    Hi, Robbo, and welcome to EL&U. You might be interested in our sister site, ELL, which is a good site for basic English questions. – anongoodnurse Sep 15 '14 at 16:52
  • No, to is not a preposition here. – tchrist Sep 15 '14 at 23:56
  • @tchrist Does some grammar rule that can help me in understanding when "to" is used as preposition and when "to" is used as part of the infinitive exist? – Robbo Sep 16 '14 at 1:20

Because that is the way English is.

I'm sorry, but that is the whole of the answer.

You can discuss the historical development, compare different constructions, but there is no known way of predicting which of the possible constructions happen to be grammatical at the present stage of the English language.

  • I hoped in some sort of grammar rule that could help me in understanding in which cases "to" is a preposition and in which cases "to" is part of the infinitive. – Robbo Sep 16 '14 at 1:18
  • @Robbo If the thing that follows to is the base form of the verb, than that is not a preposition. A road to nowhere is completely different from having nothing to do. – tchrist Sep 16 '14 at 1:35
  • Thanks. So my question now becomes "Is there some sort of rule that states whether after a word, in this case the adjective qualified, I should use an infinitive or the preposition to?". – Robbo Sep 16 '14 at 1:42
  • Yes. But it probably won't help you, because it's a lexical rule, that says that qualified may take an infinitive complement, or a PP (prepositional phrase) introduced by for. Every word that can take an object or complement (most verbs, and many adjectives and adverbs) has such a "subcategorisation frame". But though there are common patterns into which these fit, there is no way of predicting which frame a particular word takes. – Colin Fine Sep 16 '14 at 22:26
  • @ColinFine Thanks again. Where can I find such lexical rules? On a dictionary? – Robbo Sep 20 '14 at 0:53

Why would "qualified" take the infinitive? First of all, it isn't the only word to do so. Some verbs "take" the infinitive. That's the way it is. And English isn't alone when it comes to certain verbs "taking" the infinitive.

I suggest you read the article on "Infinitive" on Wikipedia, especially the section on "infinitive phrases and clauses". It might be instructive.

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