prepositions - What is different between "for" and "to"?

1. The place is important for/to me.


2. I changed the variable for/to the project.


3. I can do to/for you.



4. In/for/to this project, I created a new method.

Does it mean any difference?

This bothered me many times.

Wish someone can answer.

Answer

Yes, the use of 'to' or 'for' usually does change the meaning of your phrase, and the difference can be quite big.

To look at your examples:

The place is important for/to me.

Here the difference is possibly the smallest. Without having any definite answer on this, I say I feel that in this case 'to' give the phrase a bit more of a personal feel. But both forms convey the same concept.

I changed the variable for/to the project.

Here, the version with 'to' sounds simply strange to me, I would always go for 'for' if given only these choices. Maybe 'in' or 'of' the project sounds even better though.

I can do to/for you.

There is a very big difference between doing something to someone or for someone. When you do something to someone, that person is the (more or less) passive 'victim' of whatever you do. If I ask what you are doing to someone, correct answers may include acts of violence :)

Doing something for someone does not even necessarily include that person's cooperation, you can buy a ticket for someone, do a chore for someone or buy a gift for someone.

In/for/to this project, I created a new method.

Here I would never use 'to'. Whether to use 'in' or 'for' depends on whether you created the method inside the project or whether it is a new method created externally, to help with the specific project, in which case you use 'for'.