Next, let's look at what -ing means. Is running a noun or a verb? Depending on the sentence, it can be either one. Sometimes it's a verb: He is running. Sometimes it's a noun: Running is fun or We are tired of running. The rule is that you use -ing after a preposition (before calling, upon landing, without thinking).
Where it gets tricky is when -ing is after the word to. To is usually a preposition, but because of the infinitive, there is a very strong association of to + do, so let's clarify when to + doing is correct.
I look forward to + meeting them.
The thing is, to do is a word packet, but to + doing is where two phrases meet:
After a preposition, you can swap in a regular noun or a pronoun:
Notice that all of these prepositions are followed by -ing.
The last section is where there is no preposition, but there is -ing.