I am trying to figure out the particle and order of words in a sentence to that says "Someday I will go to Japan". Would you say:
私は日本がいつかに行きます。
or would I say:
日本は私がいつかに行きます。
I really want to learn the grammar and structure. if you could help me that would be great!
「いつか日本に行きます。」 sounds the most natural. You could insert a comma if you like to put in a pause:「いつか、日本に行きます。」 and it will be equally correct grammar.
Japanese does not usually need or use a pronoun; rather, the pronoun is, in most cases, implied. If you do not include 「私は」, it is clear to the listener that you must be talking about yourself since you did not refer the sentence to any other subject (if you were talking about someone else, you would clearly indicate that either by gesturing toward the person in question or by inserting their name or a pronoun to refer to him/her. Without such a context, the listener will easily understand you are talking about yourself). If you include 「私は」, it is not grammatically incorrect, but it sounds unnatural and the listener will know at once that you are a beginner language learner of Japanese.
「いつか」 should come first in the sentence because it is not the main point (which is 「行きます」); like in your English sentence, you are tacking it on to the front of the sentence, which would be a complete sentence without it.「日本に」 comes next to 「行きます」 because it is directly modifying the verb 「行きます」. In casual speech, it is possible to switch the sentence order and say 「日本に、いつか、行きます。」or 「いつか行きます、日本に。」, but this shows that you did not completely form the sentence in your mind before speaking (off-the-top-of-your-head, on-the-fly style of talking). Japanese can be grammatically correct in a fair number of sentence orders, but there is the most standard and formal structure versus ones which are only for talking with your peers.
「に」 is the correct particle because you are going to Japan, and to/toward is a major meaning of 「に」.「が」 is not the correct particle because that would mean that Japan is doing something (in this case, going).
Basically, 「日本は」 and 「日本が」 would tell the listener than Japan is doing something or that something about Japan itself is going to be said. Your sentence intends to be about what you are going to do, not about what Japan is going to do, so 「が」 is not applicable.「日本は私がいつかに行きます。」 is not grammatical. If it instead said, 「日本が行きます。」or 「日本は、いつか、行きます。」, this would mean, "Japan is going" or "Someday Japan will go", respectively, because the 「が」 or 「は」would show that Japan is doing something. If it said,「日本は私にいつか行きます。」, this would mean "Japan is someday going (coming) to me" --- which makes no sense, but grammatically it would be conveying something, since the 「に」 would indicate going toward.
Let's start with the verb "to go", 行く.
Here is a perfectly valid sentence:
行く。
What does it mean?
Well, this verb can take some arguments, such as
You may have noticed that no actor was listed in the sentence, despite me claiming it is "required".
It turns out that the actor is still "in the sentence", it's just omitted from the sentence's surface form. But the key point is that it's impossible to interpret the sentence without an actor in mind -- usually it defaults to "I" if there's nothing in context to suggest who the actor is.
So, a reasonable gloss for this sentence is
行く。
" (I) will go."
To explicitly state the actor, you mark him/her with が and then put it at the start of the sentence:
ジョンが行く。
"John will go."
However, in Japanese, if John (in this example) was already the topic of discussion, you need to replace が with は:
ジョンは行く。
"John will go."
(An example dialogue where John is already the topic of discussion: 「ジョンは行く？」"Will John go?" 「ジョンは行く。」 "John will go."）
(An example dialogue where John is not already the topic of discussion: 「誰が行く？」"Who will go?" 「ジョンが行く。」 "John will go."）
The は↔が stuff is complicated and hard to cover in full detail so I'll leave it there.
To specify the destination, you mark it with に and add it to the left of the verb.
日本に{LLL}行く。
"(I) will go [to Japan]{LLLLLLLL}."
We can of course combine this with the actor.
ジョンは日本に{LLL}行く。
"John will go [to Japan]{LLLLLLLL}."
In addition to arguments to the verb, there are other things you can add which affect the verb -- these are called "adjuncts".
いつか is an example of an adjunct, which means "someday".
The natural place for this particular adjunct is between the actor and the destination:
ジョンはいつか日本に{LLL}行く。
"John will go [to Japan]{LLLLLLLL} someday."
That spot is also the location for many other adjuncts...
ジョンはお父さんと日本に{LLL}行く。
"John will go [to Japan]{LLLLLLLL} with his father."ジョンは絶対に日本に{LLL}行く。
"John will definitely go [to Japan]{LLLLLLLL}."
To make the sentence polite, you turn the verb into its continuative form and then add ます.
ジョンはいつか日本に{LLL}行きます。
"John will go [to Japan]{LLLLLLLL} someday."
A note on 「私は」: since you can omit the subject of verbs in Japanese and have it inferred, often that is done. Having 「私は」 doesn't sound terrible in this particular sentence when it's standing by itself, but if you're already talking about yourself in a previous sentence it'd probably be best to drop it.
（私は）いつか日本に行きます。
There are more ways to arrange this sentence, such as
日本には（私が）いつか行きます。
Basically, the important arrangements to understand are the ones which change what element is being marked with は (in this arrangement, the destination instead of the actor), because marking the right thing is important for having the sentence flow properly in the discourse.
There are other sorts of things you can do, like move elements in the sentence around without changing whether they are marked by は or not (for example, you can say 「日本にいつか行きます」 but it's meaning is honestly not so different from the standard version). This is called "scrambling".
Unfortunately covering this all is a book.
Since that was a little unordered, let me give you a model to think about this more structurally.
The underlying sentence is
私がいつか日本に行きます。
Then you mark the right element of the sentence with は based on what is the topic of the discourse. Then you possibly drop that topic element if it feels inferrable enough. Then you possibly reorder elements around depending on scoping concerns or for pragmatic reasons (like adding on something for clarification when you notice what you said was ambiguous).
Not what you were asking about, but 「いつか日本に行きます。」 sounds a little to definite to me compared to the English sentence (which is softened by "someday" more than the Japanese sentence is softened by 「いつか」).
I think I'd opt for
いつか日本に行きたいです。
or
いつか日本に行きたいと思っています。
"I'd like to go to Japan someday."
Even more not what you were asking about, but this sentence would flow much better if it wasn't such a standalone statement... like
日本語が好きで、いつか日本に行きたいと思っています。
"I really like Japanese and would love to visit Japan someday."
or something like that. Series of short statements sounds even less natural in Japanese than in English, I think.
