Learn Hardcore Japanese: yuube no sara wo densirenzi no ue ni oita mama wasurete simaimasita. - I forgot last night's plates left on top of the microwave.

You cannot say 昨夜皿 in Japanese. Unlike English where you can sometimes stack nouns (“night dishes”), Japanese almost always needs の between two nouns when one modifies the other.

You could also say 昨日の皿 or 昨夜の皿, both meaning “yesterday’s / last night’s dishes”, with slight nuance differences in word choice (see another question on 昨夜 below).

How do you read 昨夜, and is it the same as “last night” in English? Are there other common ways to say it?

昨夜 has two readings:

ゆうべ – common, conversational, feels natural in everyday speech
さくや – more literary/formal, often seen in writing, news, or set phrases

In this sentence, it’s very natural to read it as ゆうべ.

Other common ways to say “last night”:

昨日の夜 (きのうのよる) – “yesterday night,” neutral and common
昨晩 (さくばん) – a bit more formal than ゆうべ, used in speech and writing

All of these can refer to the same time period; the differences are nuance and formality, not meaning.

What is the role of を on 皿を? Is 皿 the thing being forgotten or the thing being put?

In 昨夜の皿を電子レンジの上に置いたまま忘れてしまいました, the を belongs to the verb 置いた (“put/placed”), not to 忘れてしまいました.

The structure is:

皿を電子レンジの上に置いたまま = “(having) put the dishes on top of the microwave and left them like that”
忘れてしまいました = “(I) completely forgot (about it / about them).”

So:

皿を置いた = “(I) put the dishes (somewhere).” → 皿 is the direct object of 置く.
忘れる in this sentence has its object omitted (understood as “it” / “that situation”).

If you said 皿を忘れてしまいました, that would mean “I forgot the plate(s)” (e.g., I forgot to bring them), which is a bit different from “I forgot that I had left them on the microwave.” The original sentence focuses on forgetting the situation/state, not just the physical object.

What does 電子レンジの上に mean exactly? Is の showing possession or location?

電子レンジの上に means “on top of the microwave”.

Breakdown:

電子レンジ = microwave
〜の上 (うえ) = “the top of ~ / the surface of ~”
〜に = location marker (“in/at/on” depending on context)

So:

電子レンジの上 = “the top of the microwave”
電子レンジの上に置く = “put (something) on top of the microwave”

Here の links 電子レンジ and 上 into a single noun phrase (“the top of the microwave”), not possession in the sense of ownership, but more “the top that belongs to / is part of the microwave.”

Why is it 上に and not 上で or 上へ?

Each particle gives a different nuance:

上に – location where something is/ends up
- 電子レンジの上に皿を置く = put the plate on top of the microwave
上へ – direction towards the top (emphasis on movement)
- 上へ投げる = throw upwards
上で – place where an action happens or a condition for another action
- 机の上で勉強する = study on the desk
- よく考えた上で決めます = I’ll decide after thinking carefully

In this sentence you want to say “put it on top of the microwave (and left it there),” so 上に (final location) is the natural choice.

What does 置いたまま mean, and how does まま work with verbs?

置いたまま means roughly “having left (it) placed” or “leaving it there as it was put”.

Structure:

置く = to place / put
置いた = past form: “placed / put”
まま = “as (it is), without changing,” “in that state”

So:

置いたまま = “with it left in the state of having been placed”

In general, V-た + まま expresses:

“after doing V and leaving it in that state (without changing it).”

Examples:

電気をつけたまま寝た。
→ I slept with the light left on.
靴を履いたまま部屋に入った。
→ I went into the room with my shoes still on.

In your sentence, 皿を電子レンジの上に置いたまま = “(I) left the dishes sitting on top of the microwave.”

Why is the verb before まま in past tense (置いたまま) if the state is ongoing?

The た-form + まま does not mean the action is finished and gone; it means:

“the state that resulted from that action continues unchanged.”

So:

置いたまま = the dishes were put there at some point, and they stayed there.

You use:

V-た + まま when you’re talking about a state created by a completed action that remains:
- ドアを開けたまま出かけた。
  → I went out with the door left open.
N のまま / Adj のまま when describing a state directly:
- 子どものまま = still a child
- きれいなまま = remaining beautiful

So the past tense is about when the action happened, and まま tells you that its result is still true.

What’s the difference between 置いたまま忘れてしまいました and 置いて忘れてしまいました?

Both are understandable, but the nuance differs:

置いたまま忘れてしまいました
→ Emphasis on leaving the dishes there in that state.
Rough idea: “I forgot (about it) with the dishes left on top of the microwave.”
置いて忘れてしまいました
→ Simply lists two actions in sequence: “I put (them) there and then forgot (them / about them).”
It doesn’t highlight the ongoing unchanged state as clearly.

V-て just connects actions: A and then B.
V-たまま focuses on the continuing result of A when B happens.

In this context, 置いたまま sounds more natural and expressive for “I forgot it there (left as it was).”

What nuance does 忘れてしまいました have compared to just 忘れました?

〜てしまう adds two main nuances:

Completion – doing something completely / fully
Unintended / regretful tone – “end up doing,” often with a hint of “oops” or “unfortunately”

So:

忘れました。
→ I forgot. (neutral statement of fact)
忘れてしまいました。
→ I ended up forgetting / I completely forgot.
Often implies: “I didn’t mean to; I feel bad about it” or it was a mistake.

In this sentence, 忘れてしまいました naturally suggests regret or carelessness: you left last night’s dishes on the microwave and (regretfully) forgot about them.

Why isn’t the thing forgotten stated explicitly? Shouldn’t it be something like 皿を忘れてしまいました?

Japanese often omits information that is clear from context. In this sentence:

The important new information is that the dishes were left on the microwave.
What you “forgot” is already obvious: you forgot about that fact/situation.

The structure is:

[昨夜の皿を電子レンジの上に置いたまま] 忘れてしまいました。

A natural, fully explicit version could be:

昨夜の皿を電子レンジの上に置いたまま、そのことを忘れてしまいました。
→ “…and I forgot about that.”

But そのことを (“that thing/fact”) is usually dropped because it’s redundant.

If you said 皿を忘れてしまいました, listeners might picture you leaving the plates behind somewhere (e.g., at a restaurant) rather than forgetting that they were sitting on the microwave.

Why isn’t there a は or が for the subject (like 私は)? Who is doing the forgetting?

In natural Japanese, the subject is often omitted when it’s obvious from context. Here:

The verb is 忘れてしまいました in ます-form, which is how you talk about your own actions in polite speech in most contexts.
In a typical narrative like this, the default assumption is that “I” am the one who forgot.

A fully explicit version would be:

私は昨夜の皿を電子レンジの上に置いたまま忘れてしまいました。

But 私は is almost always dropped in such sentences unless there’s a contrast or special emphasis (e.g., “I (as opposed to someone else) forgot…”).

So, the missing subject is inferred to be “I”, and its omission is normal.

Can まま be used with other parts of speech, not just verbs like 置いたまま?

Yes. まま is a general “unchanged state” word and can combine with:

Verb (past) + まま
- 電気をつけたまま = with the light left on
- 座ったまま = while remaining seated
Noun + の + まま
- 子どものまま = still as a child / remaining a child
- パジャマのまま出かけた。
  → I went out still in my pajamas.
い-adjective (dictionary form) + まま
- 寒いまま = while still cold / remaining cold
な-adjective + な + まま
- きれいなまま = still pretty / remaining clean

The key idea is always: “in that state, without changing it.”
In your sentence, 置いたまま is “with it left in the state after being put there.”