To as a marker for an infinitive vs. to as a preposition



How is a student to know whether to is a marker for an infinitive or is a preposition? So often, to introduces the infinitive form of the verb, so it is not surprising that students want to say "I look forward to go to the beach," or "Mary is committed to finish the job on time."

Similarly, it is easy to confuse used to as in "Bob used to live in Alaska," and "Bob is used to living in a cold climate."

Dorothy Avondstondt


If you are reading some English, you can usually see the answer to this question. If to is followed immediately by a simple verb, it is part of an infinitive. If to is followed by a noun construction, it is a preposition. That's the easy and recognizable part.

But then you come across something like used to in sentences like these:

(a) When I was a student, I used to cook my own food.

(b) Now I am used to cooking for a big family.

In sentence (a), used is a past tense verb followed by an infinitive, "to cook." In sentence (b), am used to is a phrasal verb that ends with a preposition, so it is followed by a verbal noun (gerund) "cooking." One way to determine the kind of structure is to substitute the word something when a noun follows a preposition. So, we can say "I am used to something" in (b), but we can't say "I used to something" in (a). In (a) we would have to say "I used to DO something," and that means we have to substitute a verb in place of do.

In your question, you can substitute something in your examples:

(c) I look forward to something. (going to the beach)

(d) I am used to something. (playing soccer)

Therefore, a verbal noun or gerund correctly follows a preposition in your sentences.



What is more difficult, of course, is to habituate your students to using the appropriate construction after to. To go one step further than just the recognition of an infinitive or gerund form, it is a good idea, in class, to highlight the expression, verbally and on the board, whenever it comes up. Then, students can hear and see the structure in question. You can use cross-outs and Xs for the incorrect form, and write the correct one. Some teachers use color-coding in writing the right versus the wrong constructions on the board.

About used to and be used to:

Quirk et al. (A Comprehensive Grammar of the English Language. Longman, 1985) calls used to-to express past customary activity-a "marginal modal," and Collins COBUILD English Dictionary (HarperCollins, 1995) calls used to in this usage a "phrasal modal." This used to is a unit that is followed by the bare infinitive (the infinitive without to), also called the simple form of the verb.

Teaching this point along with the simple past tense, and/or the past perfect, is one way to present it to students. Teaching it alone, first, not in contrast to be used to is a good idea, I think. Then, teaching be used to at another point, perhaps as a synonym for be accustomed to, is desirable. When both uses of used to have been addressed, there is another good opportunity to use color coding on the chalkboard to differentiate the confusing forms of used to.


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