There is one very simple rule about prepositions. And, unlike most rules, this rule has no exceptions.

Rule
A preposition is followed by a "noun". It is never followed by a verb.

By "noun" we include:

• noun (dog, money, love)
• proper noun (name) (Bangkok, Mary)
• pronoun (you, him, us)
• noun group (my first job)
• gerund (swimming)

A preposition cannot be followed by a verb. If we want to follow a preposition by a verb, we must use the "-ing" form which is really a gerund or verb in noun form.

Quick Quiz: In the following sentences, why is "to" followed by a verb? That should be impossible, according to the above rule:

• I would like to go now.
• She used to smoke.

Here are some examples:

Answer to Quick Quiz: In these sentences, "to" is not a preposition. It is part of the infinitive ("to go", "to smoke").