This police force became important in the 1800s.

Is the subject of the sentence police force or force?

Also,

Mounties have become an important part of Canadian life.
What is the PN or PA of the sentence? Is it PA = important?
Or PN = part?

1. Either "police force" or "force" could be the subject here. Because "police" isn't capitalized in this sentence, I'd use just "force" as the subject, though.

2. PN — "part"
("important" would be the PA if the sentence read like this: "Mounties have become important in Canadian life." But you can see it isn't!)

Thank you so much for a quick response. I am a parent correcting my son's homework and I just had a couple I wasn’t quite certain about. It’s been awhile and I feel like I’m not really helping if I don’t even know the correct answers. This is an excellent resource. Thanks again!

You're very welcome!

In the first sentence, "police force" is the subject of the sentence. To determine the subject, you can ask "Who or what became important in the 1800s?" The answer to this question is "police force."

For the second sentence, "part" is the predicate nominative (PN) as it renames or describes the subject "Mounties." The subject "Mounties" is a plural noun referring to members of the Royal Canadian Mounted Police.

Therefore, in the sentence "Mounties have become an important part of Canadian life," "part" is the PN, not the PA. The PA (predicate adjective) would typically describe the subject and usually comes after a linking verb, which is not the case in this sentence.