spot the error!

Seen at ROK Drop:

I wonder if he volunteered for this unit or the ROK military put him in it for publicity reasons?

Highlight the space between the brackets to see the explanation.

[If a sentence begins with "I wonder if/whether..." or "He asked if/whether...", it's making a statement via indirect discourse, not asking a question.

WRONG: I wonder where he went?
RIGHT: I wonder where he went.

When you say "I wonder," you're declaring your mental state.

And what's "indirect discourse"? It's when an utterance is embedded in another utterance. "Direct discourse" involves direct quotes. Here are some examples:

DIRECT: He said, "Sit down."
DIRECT: She wondered, "Would the party be fun?"
DIRECT: He asked, "Are you Sarah Connor?"

DIRECT: "Stay away from that dildo," he advised.

INDIRECT: He told her to sit down.
INDIRECT: She wondered if/whether the party would be fun.
INDIRECT: He asked if/whether she was Sarah Connor.

INDIRECT: He advised her to stay away from that dildo.]

A deeper dive on the if/whether question is here.