Learn English – What word describes the action of changing the grammatical person of a sentence

Your question is somewhat misleading, and may contain a false assumption. A statement like the following:

Customers who have never paid a dime

is not a sentence. It's a noun phrase. Let's break that down:

Customers is a noun. Nouns are not inherently subjects or objects, so it's not correct to refer to this as either the subject or the object.

The remaining part who have never paid a dime is a relative clause, which is an embedded sentence that is used to modify something in the larger context. The subject of the embedded clause is the relative pronoun who, and the remainder have never paid a dime is the predicate of the relative clause.

However, using terms such as "predicate" and "relative clause" may be intimidating to people using your software. My best offering for newbie-friendly terminology would be:

• Call Customers a noun (most people at least know what that means)
• Call who a "question word", and make it clear by example what you're referring to
• Call have never paid a dime a "description"

I think you might be relying on gut instinct too much.

You can't know everything, let alone what he's saying.

In English, unlike languages that have methods of indicating and dealing with stative and dynamic verbs, there is little need for distinction. To complicate things further, English can approximate the "inchoative aspect", which indicates a transition into a state of existence.

All at once, she understood what her mother had always been saying.

The inchoative often has helping words like "suddenly" to indicate the transition into the state. This would explain sentences like:

I can't begin to understand what he's saying about the inchoative aspect.

or more simply:

I can't understand what he's saying.