keiyakukikan ha nagai.

契約期間 is a compound noun made from:

• 契約 = contract
• 期間 = period, term, duration

Together, they mean contract period or contract term.

In Japanese, it is very common to combine nouns like this into one larger noun, so 契約期間 functions as a single noun in the sentence.

は marks 契約期間 as the topic of the sentence:

• 契約期間は長い。 = As for the contract period, it is long.

This often sounds like you are making a general statement about it, or possibly contrasting it with something else.

If you said 契約期間が長い, that would focus more directly on the contract period as the thing that is long, often in a more neutral or descriptive way.

Very roughly:

• は = speaking about the contract period
• が = identifying the contract period as the thing that is long

Both can be natural, but は often has a more topic-comment feel.

Japanese sentences often place the main predicate at the end. Here, 長い is an i-adjective and it works as the predicate of the sentence.

So the structure is:

• 契約期間 = contract period
• は = topic marker
• 長い = is long

This is a very normal Japanese sentence pattern: [topic] は [adjective]

For example:

• この道は長い。 = This road is long.
• 夏は暑い。 = Summer is hot.

No. 長い is an i-adjective, not a verb.

However, in Japanese, an i-adjective can act as the main predicate of a sentence by itself, so you do not need a separate word for is in plain style.

That is why:

• 契約期間は長い。

already means:

• The contract period is long.

There is no missing is in Japanese here.

Because this sentence is in plain style.

• 契約期間は長い。 = plain / casual / neutral written style
• 契約期間は長いです。 = polite style

Both are correct. The version without です is common in plain speech, writing, explanations, headlines, and dictionary-style examples.

So です is not required; it just changes the politeness level.

Yes, absolutely.

• 契約期間は長い。 = plain
• 契約期間は長いです。 = polite

The meaning is basically the same. The choice depends on the situation:

• plain style: casual conversation, notes, internal narration, informal writing
• polite style: speaking to customers, coworkers in formal contexts, teachers, strangers, etc.

Here, 長い refers to duration, not physical length.

Japanese uses 長い for both:

• something physically long
• a long period of time

Since 契約期間 means a time period, 長い naturally means lasting a long time or having a long duration.

So here it means something like:

• the contract term is long
• the contract period is lengthy

Yes, it is natural and grammatically correct.

That said, depending on context, Japanese might also use more specific expressions such as:

• 契約期間が長い
• 契約期間は長めだ = the contract term is rather long
• 契約期間は長すぎる = the contract term is too long

But 契約期間は長い。 is perfectly normal as a simple statement.

Japanese does not use articles like English does.

So 契約期間 can mean:

• the contract period
• a contract period
• contract duration / contract term

The exact meaning depends on context.

This is very common in Japanese: nouns often appear without words equivalent to a or the.

Sometimes, yes.

Because は can create a contrastive nuance, the sentence can sometimes feel like:

• As for the contract term, it’s long
  possibly implying that something else is not long.

But this depends on context and intonation. It does not always have a strong contrast. Often it is just a simple topic marker.

So:

• in isolation: probably just The contract term is long
• in context: it may suggest contrast, such as with price, waiting time, or another plan’s contract term

Yes.

When an i-adjective comes before a noun, it modifies that noun directly:

• 長い契約期間 = a long contract period

Compare:

• 契約期間は長い。 = The contract period is long.
• 長い契約期間 = a long contract period

So the same adjective can be used either:

1. predicatively at the end of the sentence
2. attributively before the noun

It is pronounced:

• けいやくきかん
• keiyaku kikan

Breakdown:

• 契約 = けいやく
• 期間 = きかん

And the full sentence is:

• けいやくきかん は ながい。

No, not in ordinary Japanese writing.

Normally it would be written as:

• 契約期間は長い。

Spaces are often added in learning materials to make the structure easier to see:

• 契約期間 / は / 長い

But standard Japanese usually does not put spaces between words like English does.