Learn Hardcore Japanese: tukue no ue ni ookii nimotu ga aru to zyama desu. - When there is a big piece of luggage on the desk, it is in the way.

So here の is not “possessive” in the strict sense, but an attributive linker: 机の上 = “the upper part that belongs to/relates to the desk” → the desk’s top.

Why is に used after 上 (机の上に) instead of something like で, and why not just 机に?

Two things are going on:

上 vs. 机 by itself
- 机 just means desk (as an object).
- 机の上 means the top/surface of the desk.
When talking about where something is placed on the desk, Japanese usually uses Xの上 rather than just X.
So 机に荷物がある is not natural for “there is luggage on the desk”; you want 机の上に荷物がある.
に vs. で
- に after a place word marks a location where something exists or ends up.
  - 机の上に本がある = There is a book on the desk.
- で after a place word marks where an action happens.
  - 机の上で勉強する = To study on the desk.

In this sentence we are stating existence: 大きい荷物がある (there is big luggage), so we use 机の上に (location of existence), not 机の上で (place of an action).

Why does 荷物 take が instead of は (大きい荷物がある)? Could it be 大きい荷物はある?

Here, が marks 大きい荷物 as the thing that “exists” in that location:

X に Y がある is the standard pattern for “there is Y in/at/on X”.

So:

机の上に大きい荷物がある
= On the desk, there exists big luggage.

が often introduces new or specific information: what is there? → 大きい荷物が.

Using は would change the nuance and often the structure:

机の上に大きい荷物はじゃまです。
- “As for big luggage on the desk, it’s in the way.”
- Here は makes 大きい荷物 the topic; we are not talking about existence anymore, just making a general statement about that kind of thing.
If you leave ある in and say 机の上に大きい荷物はあるとじゃまです, it sounds awkward or odd in everyday speech.
A more natural contrastive use of は with this idea would be something like:
- 机の上に小さい荷物ならいいですが、大きい荷物があるとじゃまです。
  (Small luggage on the desk is OK, but when there is big luggage, it’s in the way.)

So, for the “there is …” pattern, が is what you should normally use.

Why is ある used instead of いる for 荷物?

Japanese has two main “existence” verbs:

ある – used for inanimate things (objects, places, abstract things)
いる – used for animate beings (people, animals; often things treated like living beings, e.g. pets)

荷物 (luggage) is not a living thing, so you use ある:

机の上に荷物がある = There is luggage on the desk.
教室に学生がいる = There are students in the classroom.

Polite forms:

ある → あります
いる → います

So the choice ある vs. いる depends on what exists, not on politeness.

The sentence has ある (plain form) but ends with です (polite). Isn’t mixing plain and polite forms wrong?

It’s normal here, because ある is in a subordinate clause, and じゃまです is in the main clause.

The structure is:

[机の上に大きい荷物がある] と [じゃまです]
- Left side (before と) is a condition clause.
- Right side (after と) is the main statement.

In Japanese, it’s very common to use plain (dictionary) form in subordinate clauses, and then choose the politeness level only for the main clause:

雨が降ると、出かけません。
(plain 降る
- polite 出かけません)
時間があるから、行きます。
(plain ある
- polite 行きます)

So 机の上に大きい荷物があるとじゃまです is perfectly natural:

Condition: …がある (plain)
Main comment: じゃまです (polite)

Using ありますと is grammatically possible in very formal written Japanese, but for everyday use, あると + polite main clause is the standard pattern.

What is と doing here? I thought と meant “and” or marked quotations. How does it mean “when / if” in this sentence?

と has several functions. In this sentence, it is the conditional と, which links a condition to an automatic or natural result.

Pattern:

[Plain-form clause] + と + [result]

Here:

机の上に大きい荷物がある (there is big luggage on the desk)
と (then/when/if)
じゃまです (it’s in the way)

So the meaning is:

“When/If there is big luggage on the desk, (then) it’s in the way.”

Typical uses of conditional と:

春になると、暖かくなります。
When it becomes spring, it gets warm.
ボタンを押すと、ドアが開きます。
If/when you press the button, the door opens.

Nuance: と-conditional often describes general truths, habits, or automatic results, rather than one-time, voluntary actions. That’s why it fits well here: “Whenever there is big luggage on the desk, it’s a nuisance.”

This is different from:

と between nouns: リンゴとバナナ (apples and bananas)
と as a quotation marker: 「行こう」と言った (said “Let’s go”).

In じゃまです, what is the subject? There’s no “it”. What exactly is “in the way”?

Japanese often omits pronouns like “it,” “he,” “this” when they are clear from context.

In this sentence:

The condition clause 机の上に大きい荷物がある sets up a situation.
Then じゃまです comments on that situation.

So the implied meaning is:

“In that situation (where there is big luggage on the desk), it (that/luggage/situation) is in the way.”

If you want to make the subject explicit, you could say:

机の上に大きい荷物があると、それがじゃまです。
When there is big luggage on the desk, that is in the way.

But in natural Japanese you usually just say じゃまです and let the listener understand from context that it’s the presence of the big luggage on the desk that is a nuisance.

Is じゃま a noun or an adjective? Why is it じゃまです, and can I say じゃまな荷物?

じゃま (usually written 邪魔) can function as both:

A noun meaning “obstruction, hindrance, nuisance”
- 邪魔をする = to get in someone’s way / to bother
  - 勉強の邪魔をしないでください。
    Please don’t disturb my studying.
A な-adjective meaning “in the way / obstructive / annoying”
- 邪魔な荷物 = luggage that is in the way
- ここは邪魔です。 = This spot is in the way.

In じゃまです, it’s being used as a predicate like a な-adjective / noun + です:

じゃまです = “(It) is a hindrance / is in the way.”

And yes, you can say:

じゃまな荷物 = “obstructive / in-the-way luggage”

Because じゃま is not an い-adjective, you cannot directly put it in front of a noun without な:

✕ じゃま荷物 (wrong)
○ じゃまな荷物 (correct)

So, treat じゃま like a な-adjective when modifying nouns (じゃまな〜), and like a noun/な-adjective when used with です (じゃまです).

Could I say 大きな荷物 instead of 大きい荷物? Is there a difference between 大きい and 大きな here?

Both are correct; the difference is small:

大きい荷物
大きな荷物

大きい is the regular い-adjective form.
大きな is a special attributive (before-a-noun) form that some adjectives have.

Nuance:

In modern everyday Japanese, 大きい荷物 and 大きな荷物 usually mean the same thing: big luggage.
大きな can sound:
- slightly more literary, story-like, or emotional in some contexts, or
- just a stylistic choice with no strong difference.

You can safely use 大きい荷物 everywhere.
If you see or want to use 大きな荷物, that’s also perfectly fine; it does not change the core meaning in this sentence.

Can this idea be expressed in other ways, like 机の上に大きい荷物があるとき、じゃまです or 机の上の大きい荷物はじゃまです? How do these differ from the original?

Yes, there are natural variations, each with a slightly different feel.

机の上に大きい荷物があるとじゃまです。 (original)
- “When/If there is big luggage on the desk, it’s in the way.”
- General rule: whenever this condition is true, it’s a nuisance.
机の上に大きい荷物があるとき、じゃまです。
- Literally: “At times when there is big luggage on the desk, it’s in the way.”
- Very close in meaning; とき makes the “time” aspect explicit.
- Slightly more “when (that happens)” than “whenever/as a rule”, but in many contexts they’ll be understood the same.
机の上の大きい荷物はじゃまです。
- “The big luggage on the desk is in the way.”
- This is more of a direct statement about the current or specific luggage, not a general conditional rule.
- は here marks 大きい荷物 as the topic: “As for the big luggage on the desk, it’s in the way.”
机の上の荷物が大きいと、じゃまです。
- “If the luggage on the desk is big, it’s in the way.”
- Focus is on the size of the already-known luggage, with the idea that if it’s small maybe it’s OK, but if it’s big, it’s a problem.

All of these are grammatical; the original sentence emphasizes a general condition-result relationship (“whenever there is big luggage on the desk, it’s in the way”), which is why it uses あると.