There are 9 parts of speech or word classes in English language:
– nouns
– pronouns
– determiners
– verbs
– prepositions
– adjectives
– adverbs
– conjunctions
– interjections

Prepositions function to show:
– relationship in space,
– relationship in time,
– relationship in logic
between two nouns (people, things, places)

The book is on the table.
Preposition ‘on’ in the above sentence expresses a relationship in space (location) between noun ‘the book’ and noun ‘the table.’

We will meet in November.
Preposition ‘in’ in the above sentence expresses a relationship in time between noun ‘we’ and noun ‘November.’

I sent the information by email.
Preposition ‘by’ in the above sentence expresses a relationship of method between noun ‘the information’ and ‘email.’

Prepositions are usually used in front of:
– nouns
This book is for John.
Preposition ‘for’ is in front of noun ‘John.’

– noun phrases (phrases that function as nouns)
This book is for your sister.
Preposition ‘for’ is in front of noun phrase ‘your sister.’

– pronouns
This book is for you.
Preposition ‘for’ is in front of pronoun ‘you.’

If a preposition is followed by a verb, the verb must be in -ing form.
Thank you for saving my life.
Thank you for save my life. (wrong)
Thank you for saves my life. (wrong)
Thank you for saved my life. (wrong)

After talking to the costumer, he went to the meeting.
After talk to the costumer, he went to the meeting. (wrong)
After talks to the costumer, he went to the meeting. (wrong)
After talked to the costumer, he went to the meeting. (wrong)

We use ‘at’ for specific locations.
at home
at my desk
at the bus stop

We use ‘at’ for companies/workplaces.
at a hospital
at Microsoft
at Google

We use ‘at’ for activities involving a group or a large number of people.
at Lisa’s party
at the cinema
at town hall
at a concert

We use ‘at’ for schools or universities.
at school
at university

We use ‘at’ for specific addresses.
at 12 George’s Street
at 734 Oxford Street

We use ‘at’ for shops/restaurants/clinics.
at the bakery
at a café
at the dentist’s

We use ‘on’ for surfaces.
on the table
on the road
on the roof
on the sofa

We use ‘on’ for building floors.
on the 15th floor
on the second floor

We use ‘on’ when someone is traveling via train, bus or plane.
on a/the train
on a/the bus
on a/the plane

We use ‘on’ for right or left position.
on the right
on the left

We use ‘on’ for electronic media.
on the radio
on the internet
on TV
on a/the website
on Twitter
on YouTube
on Instagram
on Facebook

We use ‘on’ for road names.
on Oxford Street
on Fifth Avenue

We use ‘in’ for countries.
in Indonesia
in the US
in England

We use ‘in’ for cities.
in Dubai
in London
in Tokyo

We use ‘in’ for names of areas.
in Chinatown
in Little India

We use ‘in’ for containers/rooms/areas.
in a box
in the bag
in this room
in the building
in the living room
in a lift

We use ‘in’ when someone is inside a vehicle other than bus, train, or plane.
in a car
in a van
in a taxi
in a boat
in a helicopter