When to use "to+ -ing"?

I have confusion for the use of "to+ -ing"

Eg. I look forward to receive it.
I look forward to receiving it.

I know the latter one is correct... But why is it so?

What is the difference between these two and when can we use "to receiving" ?

Please help

Thanks

Here "to" is a preposition.
Here, the "to" goes along with "look forward" no matter whether a verb, a noun, or any other word follows it:

I look forward to good weather.
I look forward to going outside.
I look forward to my next vacation.
I look forward to summer.

Prepositions are always followed by the "ing" form of a verb.

Do not confuse this with the infinive ("to + verb") form of a verb, in which the "to" functions as part of the verb rather than as a preposition.

This is one of the most confusing aspects of English. Some verbs or verbal phrases are followed by the infinitive, others by the gerund. As far as I know, there is no single rule, or even set of rules, that determines which is which. You just have to memorize them. You can find lists online, though they aren't comprehensive.

Sometimes, verbs even use both:

It has started to hail.
It has started hailing.

There are some general rules you can memorize, though, that will make it a bit easier to keep track of which are which. Simone's rule about prepositions, for instance, is a heuristic you can use that covers a lot of different situations.

Yeah I will have to remember when to use "Infinive"(to+verb) and when to use "Gerund" (verb+ing)...and then gradually I will get hold of it...correct?

Thank you so much...

some grammer books will be very helpful, you'd better remeber some fixed rules.

Whenever you say youre looking forward to a verb (e.g. going out), you use "ing"
When you say youre looking forward to a noun (e.g. Christmas), then you dont.

Get used to doing sth
Be used to doing sth
admit to doing sth

You can buy an idiom book to read everyday. Try it

For me, I remember them as to + noun not to + gerund...
As gerund is used in present continuous tense so it might baffle you to think about verb when facing with "to" (which of course leads to "infinitive")

This language pattern is somewhat difficult to understand if you want to refer back to grammar rules.

I think buying an book about verb with ing and to and then read it Therefore you are not confused about how to use Gerund Infinitive