Asking questions: ○○か。VS ○○か？ VS ○○？

Question

I was wondering if I could get some insight into something that keeps popping up for me.

When I ask questions I usually write ○○か and attach a 。 at the end. I know ○○か？ is common and I also see ○○？ from time to time. But for some reason ○○か。 feels more grammatical / the "proper" way (not causal) to me. I don't have a good way to describe this feeling. That's just how I learned it when I first started studying Japanese.

On top of this, I've never really been corrected by Japanese speakers about this either up until recently. Now on more than one occasion, by different / separate people, I've been corrected about this where ○○か。 has been changed to ○○か？.

I kind of just shrugged it off as a "that's how they would write it (more causal)" correction. Until I asked a friend the question:

USJに行ったことがありますか。

Have you ever been to USJ?

This question was ultimately misunderstood. I then asked the same question again, but just added Friend Nameは at the beginning to clarify I was asking about them. (Friend NameはUSJに行ったことがありますか。)

They then told me:

？をつけないと質問って分からないよ

この文は「Friend Nameは」ってのがあるから？が無くても質問ってのはわかるけどね

If you don't attach the ? mark then I don't understand it is a question. With this sentence, because you have "Friend Nameは" and even though there is no ? mark, I understand it is a question but...

After reading this, I'm now extremely confused about asking questions as whole. Especially when thinking about the other occasions. So, as silly as it sounds:

What is the "proper" way to ask questions? What is the "proper" punction used when asking questions? What is the differences / nuances between ○○か。, ○○か？, and ○○？

---

So I went back and asked my friend about this. I'm pretty positive this isn't correct grammatically, but they seemed to understand what I meant overall:

質問にとって文のためにわかるのはなぜ「？」が必要ですか？「○○か。」と「○○か？」は違いますか？

Why is a ? mark needed in order to understand a sentence as a question. What is the difference between "○○か。" and "○○か？"?

Their answer:

日本語の疑問文だと 〜ですか？

英語の疑問文だと Are you〜？

〜ですか。で終わると疑問文として感じ取りにくい。感覚的にYou are〜に近いかな？

？が無いと納得したようにも意味が伝わることがある。

相手が「○○した事あります」って言ったことに対して、「○○した事ありますか」は質問じゃ無くて確認だったり、同意の意味と感じ取れる。別の表現だと「〇〇した事あるんですね」みたいに言い換えれる。

難しいと思うけどわかるかな？

My translation:

If it's a question (interrogative sentence​) in Japanese then "〜ですか？"

If it's a question (interrogative sentence​) in English then "Are you〜？"

If you end with "〜ですか。" then it's hard to perceive it as question (interrogative sentence​). Perhaps it's closer to You are〜 in a sense.

If there is no ? then sometimes the meaning can come across and even be understood.

In contrast with another person (party) saying ○○した事あります, ○○した事ありますか is not a question and it can be perceived as a confirmation, an agreement meaning, or something like that. If it (○○した事ありますか) were another expression, then it's similar to expressing 〇〇した事あるんですね.

I think it's difficult, but you understand right?

---

Additional follow up from my friend:

英語みたいに「Are you〜」は疑問文

「You are〜」は肯定文って明確に別れてないから？を付けた方がわかりやすいって思えばいいかな

My translation:

Similarly in English, "Are you〜" is a question (interrogative sentence​).

When it comes to "You are〜", because it's not clearly separate / distinct as an affirmative sentence​, I think it's better to attach a ? mark as an easier way to understand (that's a question). You know?

Answer 1

USJに行ったことがありますか。
Have you ever been to USJ? (polite)

This is perfectly natural and correct as a polite question, and no question mark is necessary. A question mark should even be avoided in a very formal exchange.

If someone said a question mark is necessary, you can basically ignore it. (I could be wrong, but one possibility is that your Japanese still has many errors and your friend wanted a question mark as a parsing hint.)

And did you really use ます when you asked about that sentence? A question mark is definitely necessary in a casual question without か:

USJ行ったことある？
Have you ever been to USJ? (informal)

USJ行ったことある。
I've been to USJ. (informal)

It's also possible to construct a question with か but without ます, but this sounds like a very curt question typically said by a scary commander, a stubborn dwarven blacksmith, etc.

USJに行ったことがあるか。
Have you ever been to USJ? (curt)

---

EDIT Your friend is basically trying to point out the question-mark-less version can be ambiguous because it can also be a mild confirmation/surprise marker rather than a genuine question marker. See: やっぱり <Noun> か - why is it ok to have both?

USJに行ったことがありますか。
(Oh, so) You've been to USJ. (mild confirmation/surprise)

If said in the right context by a fluent speaker, there is virtually no risk of being misunderstood because of the absence of a question mark. Nevertheless, since your Japanese still seems rough around the edges, it may be safer for you to try to add a question mark to reduce the risk of misunderstanding, as your friend says.

Answer 2

If I get a text that says USJに行ったことがありますか。 I would recognise it as a question. Now that said, among younger people it's probably a lot more common to use ？ , and on top of that, the form with ますか。 is relatively commonly used as a literal device to introduce a topic, so one might confuse it with that usage, and wait for the next content to come.

But this usage (introducing topics) would be very unusual to use in SMS texts and/or with friends, so I personally wouldn't be confused whether USJに行ったことがありますか。 is a question or not.

Maybe your friend really means to say "there is some tiny chance of confusion" by "つけないと質問って分からないよ". IMO Japanese are prone to hyperbole when it's about adhering to some pattern (e.g. my boss used to say "Eメールの冒頭に誰あてか書かないと、誰宛のEメールかわからないだろ" but of course that's not really the case - he just wanted to "help me" by exaggerating the effect).