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."
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
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
(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
(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
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.