The completion (completed / change)了 can sit right after the verb or after the object — and which you pick depends on the object's length. With a short object, either works; with a long object, put (completed / change)了 right after the verb so the listener isn't left hanging. (completed / change)了 can also chain "after doing A, then B" in a future sequence.
構文: Subject + Verb + 了 + Object · Subject + Verb + Object + 了
I我today今天saw见了an一个old friend老朋友。
I saw an old friend today.
you你get到了home家给me我call打电话。
Call me once you've got home.
Remember: short object → (completed / change)了 either side; LONG object → (completed / change)了 right after the verb (to appear见(completed / change)了一个很久没to appear见(modifier / structural particle)的old friend老朋友 — don't make the listener wait for the (completed / change)了). And V(completed / change)了Othen / just就… chains two future actions: down下(completed / change)了subject课then / just就to go去to try to find找you你 ("come find you after class").
Watch out
- With a long, modified object, (completed / change)了 after the object reads awkwardly — keep it on the verb.