gerund/particle

[sane]

I understand that “looking forward to see you” is incorrect and that it has to be “seeing”. But why? I don’t find a thorough explanation of this in any grammar book.
'It is a good way to get (or getting) old?''
Could someone please explain to me when exactly you have to add “ing” after “to”?
Thank you.

[nyota]

I'm not a teacher***

To often forms an infinitive, e.g. I want to go to bed. I need to go to the bathroom first though.

However, in your example I look forward to seeing you, to is not part of an infinitive, it's a preposition after which you can either use a noun (e.g. I look forward to your visit) or -ing.

Other expressions that you can use this way are for example:
- be used to
- object to

I'm used to going to bed late.
He objected to having to wait for me for an hour.

[sane]

Thank you.
But what about
"a good way to get (or getting) old"?
Some say both forms (get and getting) are correct here. If so, how?

[Khosro]

Thank you.
But what about
"a good way to get (or getting) old"?
Some say both forms (get and getting) are correct here. If so, how?

Just In my opinion:

1- "a good way to get old" is an odd phrase. Getting old is not something which we try to achieve! we automatically "get old".

2- But you are right. we see both "way to + infinitive" and "way to + ...ing". I think the reason is that the meaning of "way" itself is different in those two structures. In the first one it means "method", in the second is more like a road, channel.The first structure focuses on the "intention" (to + ...), the second one on the "destination" (...ing). I think the only reliable way is to see how native speakers use these structures.