All he did was talk/ to talk / talking about himself.
"All he did was talk about himself."

I came across this sentence the other day, and I found it quite confusing that people use "was talk" in a simple sentence naturally, I mean, two verbs in a sentence? It can't be...

And so I asked some native English speakers. They told me it is weird to say "All he did was to talk or talking about himself." Then I'm like, "What? I can't even get to use a sentence that seems grammatically correct?"

My question is:
1. Which grammar form is more natural to you?
2. Is it also true to you that saying "was to talk" or "was talking" in the sentence doesn't seem right?

Thank you QQ

Question

All he did was talk/ to talk / talking about himself.
"All he did was talk about himself."

I came across this sentence the other day, and I found it quite confusing that people use "was talk" in a simple sentence naturally, I mean, two verbs in a sentence? It can't be...

And so I asked some native English speakers. They told me it is weird to say "All he did was to talk or talking about himself." Then I'm like, "What? I can't even get to use a sentence that seems grammatically correct?"

My question is: 
1. Which grammar form is more natural to you?
2. Is it also true to you that saying "was to talk" or "was talking" in the sentence doesn't seem right?

Thank you QQ

Answer

I would like to refer you to this page, which should answer your question: https://english.stackexchange.com/questions/366612/all-he-did-was-ask-or-asking

Answer

Good link from Rong. This is the key section:

"The predicative complement here can only be an infinitival: All he did was ask a question. The bare infinitival is restricted to cases where the subject NP contains "do" in a relative clause, as in your example, but "to" can optionally be added: All he did was to ask a question. – BillJ Jan 4 '17 at 15:49"

In other words, when the sentence starts: 
"All [subject] does/did is/was..." the verb form is without to or -ing e.g. complain, waste my time, ask a question"

In every other verb in the subject (noun) phrase, add "to" to the complement 
e.g. "All he asked was to have a chance"