Learn Hardcore Japanese: zikan ga kawattara, apuri de sirasemasu. - If the time changes, I will let you know through the app.

If you used 時間は変わったら, it would sound odd here. は would mark 時間 as a topic (“as for the time…”), but the sentence is really just describing a straightforward event: the time changes → I’ll notify (you).

In conditional clauses like 〜たら, が is very commonly used to mark the subject of the action in that clause:

雨が降ったら、出かけません。 – If/when it rains, I won’t go out.
電車が止まったら、教えてください。 – If/when the train stops, please tell me.

So 時間が変わったら fits that same pattern.

Why is 変わったら in the past tense (た form) if it’s talking about the future?

The 〜たら form is a special conditional pattern. It uses the past tense + ら, but it does not necessarily refer to the past in time. It often means “when/if [X happens in the future]”.

Structure:

dictionary form: 変わる (to change)
past form: 変わった (changed)
conditional: 変わったら (if/when [it] changes)

In English terms, you can think of A たら B as:

“When A happens, B will happen”
or “If A happens, then B”

So:

時間が変わったら、アプリで知らせます。
≈ “If/when the time changes, I’ll let you know via the app.”

The past form is just how the たら conditional is built; it doesn’t imply the event has already happened.

Does 〜たら here mean “if” or “when”? What nuance does it have?

In 時間が変わったら、アプリで知らせます, 〜たら can be read as either:

when the time changes
or
if the time changes

The nuance depends on context:

If it is certain the time will change (e.g., a schedule is definitely going to be updated), when feels more natural in English.
If it is uncertain (maybe the time might change, maybe not), if feels better.

Japanese 〜たら itself is flexible and covers both. It just says:
“In the case that [this has happened], [that will happen].”

What’s the difference between 変わったら and 変えたら?

This is about intransitive vs. transitive verbs.

変わる – intransitive: “to change” (something changes by itself / we don’t mention who changes it)
- 時間が変わる – the time changes
変える – transitive: “to change (something)” (someone actively changes it)
- 時間を変える – (someone) changes the time

So:

時間が変わったら – “if/when the time changes” (focus on the time changing)
時間を変えたら – “if/when (I/you/someone) change(s) the time” (focus on the person doing the change)

In your sentence, it’s natural to describe the time itself as changing, so 変わったら is used.

Why is it アプリで知らせます and not アプリに, アプリから, or something else?

The particle で here indicates the means or tool used to do the action:

アプリで知らせます – “I will notify (you) by/through the app.”

Common patterns for で as a tool/means:

メールで送ります。 – I’ll send it by email.
電話で話しましょう。 – Let’s talk by phone.

Why not other particles?

アプリに知らせます – would suggest “notify the app” (the app is the receiver), which is not the intended meaning.
アプリから知らせます – could be interpreted as “I will notify (you) from the app,” but this focuses on the source rather than the tool. In practice, アプリで知らせます is the standard, natural phrasing for “via the app.”

So アプリで is the most natural choice to express the app as the means of notification.

What is the nuance of 知らせます compared with 教えます or 伝えます?

All three involve giving information, but they differ slightly:

知らせる
- Basic meaning: “to let someone know,” “to notify,” “to inform.”
- Often used for notifications, alerts, simple information updates.
- Fits very well with apps, alarms, systems, etc.
- アプリで知らせます。 – I’ll notify you via the app.
教える
- Basic meaning: “to teach,” “to tell (information someone wants/needs to learn).”
- More about teaching/explaining or giving specific information on request:
  - 時間を教えます。 – I’ll tell you the time / let you know the time.
  - 日本語を教えます。 – I teach Japanese.
伝える
- Basic meaning: “to convey,” “to pass on (a message, feelings, information).”
- Often used for relaying messages from one person to another:
  - 彼にそのことを伝えます。 – I’ll tell him that.

In app/notification contexts, 知らせます is the most natural verb. It sounds like an automated notice or straightforward alert.

Who is the subject of 知らせます? Where is “I” or “the app” in the Japanese sentence?

Japanese often omits the subject if it’s clear from context.

In 時間が変わったら、アプリで知らせます。, there is no explicit subject like 私 (I) or アプリ (the app). Possible implicit interpretations:

(私が) アプリで知らせます。 – “I will let you know via the app.”
(アプリが) 知らせます。 – “The app will notify you.”

Context decides which is more natural:

If a person (a company, service rep, teacher, etc.) is talking:
“If/when the time changes, I/we will notify you via the app.”
If it’s describing the behavior of the app:
“If/when the time changes, the app will notify you.”

Japanese does not need to state the subject each time once it’s understood; English must choose one in translation.

Why is 知らせます in the polite 〜ます form? Can I say 知らせる instead?

知らせます is the polite form (ます-form), commonly used in:

customer-facing language (apps, services, instructions)
talking to people who are not close friends/family
neutral public statements

Base verb: 知らせる
Polite: 知らせます

You can say 知らせる instead, but that’s the plain form, which:

is used in casual conversation,
appears in dictionaries and basic example sentences,
is used inside some larger sentences or in informal writing.

Compare:

時間が変わったら、アプリで知らせる。 – casual.
時間が変わったら、アプリで知らせます。 – polite, neutral; typical for instructions, UI text, or announcements.

In most learning contexts and app/system messages, the polite 知らせます is what you’d expect.

Could you change the word order, like アプリで時間が変わったら知らせます? Is that okay?

Japanese word order is flexible, especially with marked phrases (with particles). Possible variants include:

時間が変わったら、アプリで知らせます。 – very natural.
アプリで、時間が変わったら知らせます。 – also possible; slight emphasis on “via the app.”
アプリで時間が変わったら、知らせます。 – grammatically possible, but the phrase アプリで時間が変わったら can feel a bit unclear at first glance (it can sound like “if the time changes in the app…”).

The safest and most natural for learners is the original:

時間が変わったら、アプリで知らせます。

It clearly separates the condition (時間が変わったら) and the result/action (アプリで知らせます).

Does this sentence mean the app automatically notifies me, or that a person will notify me using the app?

The Japanese sentence itself is ambiguous on this point, because the subject is omitted.

It can mean:
“When the time changes, the app will automatically notify you.”
It can also mean:
“When the time changes, I / we will notify you (by sending something via the app).”

To make it explicit, Japanese could add a subject:

時間が変わったら、アプリが知らせます。 – The app will notify you.
時間が変わったら、私がアプリで知らせます。 – I will notify you via the app.

In real usage, context (who is speaking, where the text appears) tells you which meaning is intended.