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)Â (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:Â In the following sentences, why is the preposition "to" followed by a verb? That should be impossible, according to the rule that you have just read.
- I would like to go now.
- She used to smoke.
See answer
The answer is that in "I would like to go now" and "She used to smoke", the word "to" is not a preposition. It is part of the infinitive ("to go", "to smoke").Here are some examples:
| subject + verb | preposition | "noun" | note |
|---|---|---|---|
| The food is | on | the table. | noun |
| She lives | in | Japan. | proper noun |
| Tara is looking | for | you. | pronoun |
| The letter is | under | your blue book. | noun group |
| Pascal is used | to | English people. | |
| She isn't used | to | working. | gerund |
| I ate | before | coming. |
sources : Original Link