Verbs before the preposition "to"

Question

It is well known that preposition must be followed by a verb in -ing form. I do see sentences in which it really occurs, for example,

This is crucial to building a good team.

He‘s not used to driving in London

However, I also see sentences where the preposition "to" is followed by a verb which is not in -ing form, for instance

This is watered to grow faster... (I thought it should be "This is watered to growing faster..." since "to" is a preposition)

The criteria used to improve performance... (I thought it should be "The criteria used to improving performance..." since "to" is a preposition)

Answer 1

In the first case, "to" is being used a as a preposition followed by a gerund, as you state.

In the latter case, "to" is being used as part of an infinitive, in particular an infinitive of purpose (see Wiktionary).

This infinitive serves to indicate the purpose of an action. So "the criteria used to improve performance" means "the criteria used for the purpose of improving performance."

Answer 2

You gave four examples:

1. This is crucial to building a good team.
2. He‘s not used to driving in London
3. This is watered to grow faster...
4. The criteria used to improve performance...

1 = This is vital to/for building a good team. 2 = He is not accustomed to driving in London. 3 = This is watered in order to grow faster...

Please note the difference between 1 and 3: in 1 'to' is placed after an adjective whereas in 3 it appears after a verb.

Please note as well the difference between 2 and 4: in 2 we have structure 'to be used to sth', but in 4 'used to' appears after a plural noun.

In 4 'used to' may be a modal (or auiliary) verb as 'I used to run a lot when I was young' denoting a past habit (mainly in BrE. See The Oxford English Grammar, p.155 4.29 Auxiliaries [11]-[13], ISBN 0-19-861250-8 or Penguim Students' Grammar of English, p.118 10.27 Used to, ISBN 0-14-080982-1). It may also be a verb 'use' in Past Tense as in 'The hammer used to break the window ...' Without the rest of the sentence I cannot be sure how to classify 'used to' in 4.