When you have a series of ideas, actions, or items like this, you want to use the same pattern of words. Let’s look at a simple example of this:
Fred likes sailing, fishing, and sleeping.
Notice how I used the same form of the verb for each action. This is called parallel structure. Here’s how I might ruin that parallel structure:
Fred likes sailing, fishing, and to swim.
See how it reads awkwardly?
You use parallel structure in the first two actions: “stammer over the tea cakes” and “keep the butter in the refrigerator too long.” The third, though, goes off course. You can make it parallel by bringing that third in line with the other two:
She would stammer over the tea cakes, keep the butter in the refrigerator too long, and forget to put napkins in the holder.