What is the parallel structure of this sentence? To install new carpeting, workers must first remove the old carpet, prepare the surface, and then lay down the matting

It looks parallel to me.

The series has 3 parallel verb phrases in it:
~ remove the old carpet
~ prepare the surface
~ lay down the matting

The only thing I would suggest is to move "then" to the second verb phrase and then include "finally" or "last" with the third verb phrase ... like this:

~ first remove the old carpet
~ then prepare the surface
~ finally lay down the matting

The parallel structure of the sentence "To install new carpeting, workers must first remove the old carpet, prepare the surface, and then lay down the matting" is important in maintaining balance and clarity within the sentence. Parallel structure means using the same grammatical structure or pattern for similar elements within a sentence or across multiple sentences.

In this case, the sentence follows a parallel structure in listing out the steps required to install new carpeting. Each step is presented in the same verb form with the auxiliary verb "must" followed by the base form of the verb. The parallel structure of the sentence can be demonstrated by organizing the sentence as:

To install new carpeting, workers must:
- First remove the old carpet.
- Then prepare the surface.
- Finally, lay down the matting.

Notice how each step starts with a verb, follows a consistent structure with the word "must," and maintains a parallel construction by using the base form of the verb. This parallel structure helps the sentence flow smoothly and makes it easier to understand the series of actions required.