I´m not a native speaker of Tok Pisin, but I´ll give it a try:
"X has entered the room":
X i go long rum pinis.
"X has left the room":
X i kam ausait long rum pinis.
the room=rum (no article).
The "i" is the predicate marker and is dropped if X is the first or second person singular.
Although the tense is present tense, the "pinis" shows that the action of entering or leaving the room is finished. The "pinis" can also be dropped if the speaker feels that the tense is made clear through the context.
Another possibility of expressing past tense is "bin":
X bin go long rum.
X bin kam ausait long rum.
I hope I could help