Learn Hardcore Japanese: sonna toki, mukasi no doukyuusei kara messeezi ga kuru to, kyuu ni sabisisa ga kiete waratte simau. - At such times, when I get a message from an old classmate, my loneliness suddenly disappears and I end up laughing.

そのとき is more like “at that (specific) time / at that moment.” It usually points to one particular moment, almost like pointing with your finger.

So:

そんなとき → general type of situation: “When that kind of thing happens / at times like that.”
そのとき → one concrete moment: “At that exact moment.”

Why is it 昔の同級生 and not 同級生の昔? How does this structure work?

昔の同級生 is a noun + の + noun pattern:

昔 = “the past / long ago”
同級生 = “classmate”
昔の同級生 = literally “classmate of the past” → “old classmate / former classmate.”

Here, 昔 is describing 同級生, so it goes before it with の.

同級生の昔 would literally be “the past of a classmate,” which doesn’t mean “old classmate” and sounds strange in this context.

So the pattern is:

昔の友だち = “an old friend” (friend from long ago)
高校の同級生 = “a high school classmate”
会社の同僚 = “a coworker from the company”

Always [descriptor] の [main noun], not the other way around.

What does から mean in 同級生からメッセージが来る? Is it “from” or “because”?

Here から means “from” (the source):

同級生からメッセージが来る = “A message comes from a classmate.”

In Japanese, から can mean:

from (origin / source)
- 友だちからプレゼントをもらった。
  “I got a present from a friend.”
because (reason)
- 雨だから、行きません。
  “I won’t go because it’s raining.”

In this sentence, it matches usage (1): the origin of the message. There’s no “because” meaning here.

Why is it メッセージが来る and not メッセージをもらう?

Both are possible, but they focus on different things.

メッセージが来る
Literally: “A message comes.”
The message is the subject (marked by が). The focus is on the arrival of the message itself as an event.
メッセージをもらう
Literally: “(I) receive a message.”
The implied subject is I, and the focus is on me receiving it.

In this sentence, the structure is:

When a message (subject) comes from an old classmate, the loneliness disappears and I end up smiling.

Using メッセージが来る makes the first clause feel like an automatic trigger event: “Whenever such a message comes, then X happens.” That matches well with the と conditional used here.

You could say メッセージをもらうと, but the nuance becomes more about your action of receiving rather than the “message’s arrival” as a triggering event. メッセージが来る is more neutral and common in this context.

What is the function of と after 来る? Is it the quotation と?

Here, と is not the quotation と. It is the conditional “when / whenever / if” と.

メッセージが来ると = “when a message comes / whenever a message comes.”

This conditional と has a few key features:

It often expresses automatic or natural results.
It’s often used for general truths or habitual reactions, like:
- 春になると、暖かくなる。
  “When it becomes spring, it gets warm.”
- ボタンを押すと、ドアが開く。
  “If/when you press the button, the door opens.”

In your sentence, the structure is:

[メッセージが来ると], [寂しさが消れて笑ってしまう]

So:
“When a message comes, the loneliness suddenly disappears and I end up smiling.”

It’s clearly conditional, not “he said that …” or any other quoting use.

What does 急に mean here, and why can’t we just say 急?

急に means “suddenly / abruptly.”

急 by itself is a noun / na-adjective meaning “sudden; urgent.”
To modify a verb, you usually need an adverb form.

So:

急な変化 = “a sudden change” (急な modifies the noun 変化)
急に変化する = “to change suddenly” (急に modifies the verb 変化する)

In your sentence:

急に寂しさが消えて = “the loneliness suddenly disappears”

If you said 急寂しさが消えて, it would be ungrammatical. You need 急に as the adverb.

What is 寂しさ exactly? How is it different from 寂しい?

寂しい (さびしい) is an i-adjective meaning “lonely / lonesome.”

寂しさ is the noun form made by adding さ to the adjective:

寂しい → 寂しさ

This -さ ending turns many adjectives into abstract nouns that express the degree or state of that quality:

高い → 高さ = “height”
重い → 重さ = “weight”
新しい → 新しさ = “newness”
寂しい → 寂しさ = “loneliness (the feeling / quality)”

So:

寂しい = “(I am) lonely” (adjective)
寂しさ = “loneliness” (noun)

In the sentence:

寂しさが消えて = “the loneliness disappears”

You can’t say 寂しいが消えて here; you need the noun form to be the subject of 消える.

Why is it 寂しさが消れて with the て-form? What is the て doing?

消えて is the て-form of 消える (“to disappear”).

The て-form is used to link actions/states. In this sentence, it connects:

寂しさが消えて – “the loneliness disappears,” and then
笑ってしまう – “(I) end up smiling.”

So XてY here means “X and (then) Y”, often with a nuance that X leads to Y:

寂しさが消えて笑ってしまう。
“The loneliness disappears, and I (as a result) end up smiling.”

The て-form is very flexible; some common meanings when linking two clauses are:

pure sequence: “do X and then Y”
cause–effect-ish: “X happens and so Y”
simultaneous: “do X while doing Y”

Here it’s basically sequence with a light cause–effect (“once the loneliness vanishes, I smile”).

What does 笑ってしまう mean exactly? Does しまう always imply regret?

笑ってしまう is 笑う (to laugh/smile) in て-form plus しまう.

Vてしまう has a few common nuances:

Completion: “to do something completely / finish doing”
Unintended / uncontrollable: “to end up doing / can’t help but do”
Often, but not always, regret or mixed feelings

In your sentence:

笑ってしまう = “I end up smiling / I can’t help but smile.”

Context: a lonely feeling disappears because of a kind message. There’s no regret. The nuance is more:

“Before I know it, I’m smiling.”
“I just naturally end up smiling.”

Examples where しまう has regret:

お金を全部使ってしまった。
“I (unfortunately) spent all my money.”
寝坊してしまいました。
“I overslept (oops).”

So しまう does not always imply regret; it can also express a natural, unintentional outcome, which is how it’s used here.

Everything is in the non-past form (来る, 消えて, 笑ってしまう). Why is it understood as a present / habitual situation?

Japanese non-past (dictionary form) usually covers both:

present and
future / habitual meanings

The exact reading depends on context.

With the と-conditional and a general-sounding situation, non-past usually describes a habitual pattern or whenever X happens, Y happens:

朝になると、コーヒーを飲む。
“When it’s morning, I drink coffee.” (habit)
雨が降ると、道路が混む。
“When it rains, the roads get crowded.” (general pattern)

In your sentence:

メッセージが来ると…笑ってしまう。

That structure signals a general, repeated reaction: “When that sort of thing happens, this is what (typically) happens to me.”

So it’s understood as present/habitual, even though the verbs are in plain non-past form.

Why is が used with メッセージ and 寂しさ instead of は?

In this sentence:

メッセージが来る
寂しさが消えて

が is marking the grammatical subject of each verb:

メッセージ (subject) が来る (“a message comes”)
寂しさ (subject) が 消える (“the loneliness disappears”)

が often introduces new information or just straightforwardly marks who/what does the action.

は marks the topic and often contrasts or brings something into focus as “as for X…”.

If we said:

メッセージは来ると… – would feel like “As for a message, when it comes…” (you’d expect some contrast or further comparison)
寂しさは消えて… – “As for the loneliness, it disappears and…”

But here we’re simply stating events that occur:

a message (subject) comes, then
the loneliness (subject) disappears.

So が is the natural choice.

Where is “I” in this sentence? Why is there no explicit 私?

Japanese often omits the subject (like 私, “I”) when it’s clear from context.

In the part:

笑ってしまう。

There’s no 私, but it’s naturally understood as:

（私は）笑ってしまう。 = “(I) end up smiling.”

Earlier parts メッセージが来る, 寂しさが消える talk about events and feelings that involve the speaker. Since it’s clearly about the speaker’s loneliness and reaction, Japanese doesn’t need to repeat 私.

If you really wanted to include it (for clarity or emphasis), you could say:

そんなとき、昔の同級生からメッセージが来ると、急に寂しさが消えて、私は笑ってしまう。

But usually it’s left out as unnecessary.

Is this sentence polite or casual? How would you make it more polite?

The sentence is in the plain (casual) style:

来る (plain)
消えて (plain te-form)
笑ってしまう (plain)

To make it polite, you use the ます-form for the final verb (and optionally others if you wish):

Most natural polite version:

そんなとき、昔の同級生からメッセージが来ると、急に寂しさが消れて笑ってしまいます。

You could also make it a bit more conversational/politeness-softener:

そんなとき、昔の同級生からメッセージが来ると、急に寂しさが消えて、思わず笑ってしまうんです。

But the basic change is just 笑ってしまう → 笑ってしまいます.

Could we use メッセージが来たら instead of メッセージが来ると? What’s the difference?

You can say:

メッセージが来たら、急に寂しさが消えて笑ってしまう。

It’s grammatically fine and still means roughly “When a message comes, I end up smiling.”

However, there’s a nuance difference:

～と conditional:
- Often used for automatic / natural results.
- Often expresses general truths / habitual patterns.
- Feels more like a reliable trigger → result link.
～たら conditional:
- More flexible: can be “when / if / after”.
- Can be used for one-time events as well as habits.
- Slightly less “automatic law of nature” feeling.

In your sentence, メッセージが来ると fits very well because it describes a typical, almost automatic emotional reaction: “Whenever this happens, this is what happens to me.”

来たら would make it feel a bit more like “if/when that happens (on some occasion)”, slightly more event-like and less law-like, but still acceptable.

Can we change the order of the clauses, like putting the result first and メッセージが来ると later?

Not with と in the same way.

With と as a conditional, the cause/trigger clause must come before と, and the result must come after it:

✅ メッセージが来ると、寂しさが消えて笑ってしまう。
“When a message comes, the loneliness disappears and I end up smiling.”

If you tried to reverse them:

❌ 寂しさが消えて笑ってしまうと、メッセージが来る。

This would mean something different (“When my loneliness disappears and I end up smiling, a message comes”), and sounds strange in context.

You can move some phrases around inside the clauses, though:

昔の同級生からメッセージが来ると
→ メッセージが昔の同級生から来ると (still okay, just different emphasis)

But the overall structure:

[trigger clause] + と + [result clause]

must stay in that order.