To relax. Or for relaxing

To relax. Or for relaxing
 

My Sts always make the same mistakes, one of them is for plus ing instead of to plus infinitive.
For example
This room is good for relaxing myself instead of to relax myself.
Can anyone suggest a good explanation?

Both the gerund and the full infinitive (both of these acting as nouns) can be used interchangeably in most cases.

I'm certain there are English speakers that would say that "to read" is not the same thing as "reading," but when they act as a prepositional object (a noun), both the full infinitive and the gerund mean the exact same thing.

This room is good for relaxing myself instead of to relax myself.
Can anyone suggest a good explanation?

In English the gerund is usually used after some verbs, adjectives+prepositions, nouns+prepositions (no pleasure in shopping, his apology for being late)
In the above example we have the adjective 'good for' ( it makes you healthy)
and 'good at+gerund= good at skating (ability)
you are brilliant at maths
I'm hopeless at languages.

'Good for you' (makes you healthy, happy, feel right)
'Good for doing something (like relaxing or working)' (purpose)

'Good at' is used to describe ability, as you stated.

Your comment about a gerund usually appearing after some verbs is only one place where a gerund can appear. The gerund always acts as a noun. Therefore, it can appear anywhere a noun can.
In the original post, it was used as a prepositional object (which is a noun).

The full infinitive is also used as a noun in English.

I meant this screwdriver is for opening bottles and not to open botles or i used to go to the beach to sunbathe not for sunbThing. That was my question sorry if i didnt ask correctly

Any Ideas?