It says I have to choose either an infinitive or a gerund. I can't see a way to make it a to+infinitive or ing.

Please explain why.

we can not make it infintive or ing Because the sentence is continuity and we the verb doing so its impossible to gather infintive verb and ing verb together that's will be faulty arrangement

- "a.) it includes letters and books gathered over 1200 years, and consists of two parts.

- most of the new testament is letters from paul to the different churches, and since there's a new testament that means there is an old testament. "

greet them:

buenas noches.

that is considered night time which is when "good night" is used.