It's about the use of Spend and keep + ing.
When the verbs which are after keep and spend are followed by -ing and when not? I don't understand the rules
Adding -ing to an English verb creates either 1) a gerund or 2) a progressive verb form. Each has its own distinct usages.
Within the Reference section of this site, you'll find helpful definitions and examples of both gerunds and the progressive "aspect". My suggestion is that you read through each of these brief links, and then apply what you have learned to the writing of example sentences of your own construction using the verbs keep, spend, and any others that may suit your fancy.
After this exercise, please feel free to post any of your example sentences or questions related to them back to this thread.