Como usar a formação 'To + -ing'?

Fleal,

A complete guide to making your own honey wine mead?"

Quando "to" funciona como preposição, o verbo seguinte recebe ING. A melhor forma de reconhecer é com o tempo e convívio com o idioma.


"Let" não requer o uso do "to"; "allow" requer o uso do "to". Mais outro assunto que deve ser aprendido com o convívio e observação. Não recomendo sair destes padrões: "let" sem "to"; "allow com "to".

The question is complex, really. "To" is always a preposition, except in the rare cases where it acts as an adverb (such as in "When he came to, he felt better."). The verb following "to" may be in its infinitive form, or in its verbal (ing) form. Usage dictates which makes more sense in each case:

This is the key to open the front door. (but not 'opening')
This is the key to deciphering these secret messages. (or 'to decipher')

It's very hard to write a "rule" for the usages above.

Yes, 'to let', meaning 'to allow', usually doesn't need 'to' afterwards, but it does depend on (1)sentence structure and (2)the meaning of "let":
(1)Only authorized personnel are allowed in here, but I'm going to let you (in order) to show I'm not hiding anything.
(2)He has two apartments. He lives in one. The other apartment he decided to let to make some extra cash.