ferii ga kekkousita tame, nitteihenkouse zaru wo enai.

Here, ため means because of or due to, not for the purpose of.

In Japanese, ため can have more than one meaning:

• V dictionary form + ために = in order to do
• Clause + ため = because / as a result of

In this sentence, フェリーが欠航したため means because the ferry was canceled.

This use of ため sounds more formal and objective than から or ので. It is common in writing, announcements, reports, and business-style Japanese.

A less formal version would be:

• フェリーが欠航したので、日程を変更しなければならない。

Because the cancellation is treated as an event that already happened, and that event caused the next situation.

So:

• 欠航したため = because it was canceled
• not because it is canceled in a broad timeless sense

Japanese often uses the past form before a cause expression when the cause is a completed event.

So the flow is:

1. The ferry got canceled.
2. As a result, the schedule must be changed.

が marks フェリー as the thing that underwent the event 欠航した.

With 欠航する, the canceled thing is usually marked with が:

• 便が欠航した
• 飛行機が欠航した
• フェリーが欠航した

Using を would not be natural here, because フェリー is not a direct object. It is the thing that was canceled.

Could は be used? Yes, but it would change the nuance:

• フェリーは欠航したため...

This would make the ferry the topic, possibly with a contrastive feel. が is the more neutral choice for simply stating what happened.

日程変更 is a verbal noun (often called a suru-noun). That means it can function like a noun by itself, but it can also combine with する to mean to change the schedule.

So these are both possible:

• 日程変更する
• 日程を変更する

Both mean basically the same thing.

The version with 日程変更する is especially common in formal, business, or written Japanese. It sounds compact and administrative.

So in your sentence:

• 日程変更せざるを得ない

is a formal way of saying:

• 日程を変更せざるを得ない

For a learner, 日程を変更する may feel easier to understand, but the sentence as written is very natural in formal contexts.

〜ざるを得ない means:

• cannot avoid doing
• have no choice but to do
• must do, due to circumstances

So:

• 日程変更せざるを得ない = we have no choice but to change the schedule

This expression often implies that the speaker does not especially want to do it, but circumstances leave no alternative.

Compare:

• しなければならない = must do
• するしかない = there’s no choice but to do
• せざるを得ない = cannot help but do; have no alternative, often more formal

So 〜ざるを得ない has a slightly stronger sense of being forced by circumstances.

Because ざる comes from an older negative form, and する behaves irregularly with it.

With modern negative Japanese, you know:

• する → しない

But with the older negative pattern used in 〜ざるを得ない, する becomes:

• する → せず
• する → せざる

So:

• せざるを得ない is correct
• しざるを得ない is incorrect

This is just something you need to memorize as a special form.

A few useful patterns are:

• 行く → 行かざるを得ない
• 食べる → 食べざるを得ない
• する → せざるを得ない
• 来る → 来ざるを得ない pronounced こざるをえない

Yes. It sounds fairly formal, written, or serious.

You will often see it in:

• news
• reports
• official explanations
• business writing
• polite speech with a formal tone

A more casual version of the same idea would be:

• 日程を変更しなければならない
• 日程を変更するしかない

So this sentence has a somewhat official or written feel.

The subject is omitted, which is very common in Japanese.

The sentence does not explicitly say who must change the schedule. That is understood from context.

Depending on the situation, it could mean:

• we have no choice but to change the schedule
• I have no choice but to change the schedule
• they have no choice but to change the schedule

In English, you usually need to supply a subject. In Japanese, if it is obvious from context, it is often left unstated.

Yes. The original sentence is natural, but quite formal. A more everyday version would be:

• フェリーが欠航したので、日程を変更しなければならない。
• フェリーが欠航したから、日程を変更するしかない。

These mean basically the same thing, but they sound more conversational.

Very roughly:

• ため and 〜ざるを得ない = formal
• ので / から and 〜しなければならない / 〜しかない = more everyday

No. 欠航 is used for transportation services that are canceled, especially:

• flights
• ferries
• ships

For example:

• 飛行機が欠航した = the flight was canceled
• フェリーが欠航した = the ferry was canceled

For trains or buses, Japanese often uses 運休 instead:

• 電車が運休した
• バスが運休した

So 欠航 is not limited to ferries, but it is used for certain types of transport more than others.