FOR vs TO for purpose or reason

It is sometimes not clear for learners what structure to use to talk about the use of something as opposed to the reason for doing something. .

To explain the use of an object, we use the structure 'object + for + noun or gerund'
For example:
- The safety belt is for protection.
- The spray / it is for cleaning mirrors.

To explain the reason for doing something or why we use an object, we use the structure 'action verb + object + infinitive'.
For example:
- We use a safety belt to protect ourselves.
- I use the spray to clean mirrors.

• OBJECT + FOR + GERUND (verb in -ing form)
  Use or function of things/objects
  To explain the function of an object, what it is used for, we use for followed by
  a noun, or a verb in the -ing form.
  • What is this sponge for? (= What is its purpose or function?)
    It is for cleaning the sink.
  • What's this barrier for? (= What do you use it for?)
    It's for the dog.
    It's for keeping in the dog.
  • What's this remote control for? (= What is its function?)
    It's for the gates. It's for opening and closing the gates.
  • What's this knife for? (= What do you use it for?)
    It's for opening oysters.
  • What's the new suit for? (= What are you going to wear it for?)
    It's for my interview. I want to make a good impression!
  • What's that knob for?
    It's for turning on the radiator.
    
    
• ACTION VERB + OBJECT +  INFINITIVE
  Reason for doing something:
  To explain why we do something, the reason for which it is done, we use the infinitive (example: to do).
  • What do you use this sponge for? (=  Why do you use it?)
    I use it to clean the sink.
  • Why did you turn on the radio? (= What did you turn on the radio for?)
    I turned on the radio to listen to the news.
  • Why are you running?
    I'm running to catch the bus.
  • Why are you calling?
    I'm calling to see if you arrived safely.
  • Why do you study every day?
    I study to improve my level of English.
  • Why are you studying so hard?
    I'm studying hard to become a doctor.
  • Why did you go to London?
    I went to London to visit Buckingham Palace.
  • Why are you moving to the city?
    I'm moving to find a better job.
    I'm moving to look for more opportunities.