toraiarukikan ha mizikai.

A natural reading is:

トライアルきかん は みじかい。
Romanized: toraiaru kikan wa mijikai

• トライアル = trial
• 期間 = period; duration
• は is written ha but pronounced wa here
• 短い = short

So the whole sentence is read Toraiaru kikan wa mijikai.

Because トライアル is a loanword, borrowed from English trial.

In Japanese:

• katakana is commonly used for foreign loanwords
• hiragana is used for native grammar words and some native words
• kanji is used for many core content words

So in this sentence:

• トライアル is katakana because it is a borrowed word
• 期間 and 短い are native Sino-Japanese / Japanese vocabulary, so they use kanji

期間 means period, term, or duration of time.

It is read きかん.

In トライアル期間, it means trial period.

So:

• トライアル = trial
• 期間 = period
• トライアル期間 = trial period

This is a very common type of Japanese compound noun, where two nouns combine into one bigger noun.

This is one of the first big pronunciation rules learners notice.

When は is used as the topic particle, it is pronounced wa, not ha.

So in this sentence:

• written: トライアル期間は短い
• pronounced: トライアルきかんわみじかい

This is just a standard grammar rule for the particle は.

は marks the topic of the sentence.

So トライアル期間は means something like:

• as for the trial period,
• the trial period,
• speaking of the trial period,

Then 短い gives the comment about that topic:

• it is short

So the structure is:

• トライアル期間は = as for the trial period
• 短い = is short

Because 短い is an i-adjective, and i-adjectives can end a sentence by themselves in plain style.

So:

• 短い。 = is short
• 短いです。 = is short (more polite)

Both are grammatical.

This sentence is in a plain / casual / dictionary-style form:

• トライアル期間は短い。

A polite version would be:

• トライアル期間は短いです。

Because 短い is an i-adjective.

In Japanese:

• i-adjectives can act as predicates by themselves
• na-adjectives and nouns usually need だ or です when they end a sentence

Compare:

• 期間は短い。 = The period is short.
• 期間は静かだ。 = The period is quiet.
• 期間は問題だ。 = The period is a problem.

So 短い does not take だ here.

短い is an i-adjective meaning short.

Its dictionary form is 短い.
It ends in -い, which is the typical form of an i-adjective.

A few useful forms:

• 短い = short
• 短くない = not short
• 短かった = was short
• 短くなかった = was not short

In this sentence, it is in plain non-past form:

• トライアル期間は短い。
• The trial period is short.

Japanese often puts the main description at the end.

English:

• The trial period is short.

Japanese:

• トライアル期間は 短い。
• literally: As for the trial period, short.

This is very normal Japanese sentence structure:

• topic + は + comment

So although it may feel backwards at first, it is one of the most basic Japanese patterns.

Yes, you could.

Both are understandable:

• トライアル期間は短い。
• トライアルの期間は短い。

The first version treats トライアル期間 as one compound noun: trial period.
The second version is a little more explicitly the period of the trial.

In many real-life contexts, the compound version トライアル期間 sounds more compact and natural.

It depends on context.

Japanese often leaves that kind of detail unstated.

So トライアル期間は短い。 could mean:

• The trial period is short (a specific one already understood from context)
• Trial periods are short or The trial period tends to be short (a more general statement)

Context tells you which interpretation is intended.

No. Normally, Japanese is written without spaces.

So the standard writing would be:

トライアル期間は短い。

Learner materials often insert spaces to make the structure easier to see:

• トライアル期間　は　短い。

That spacing is just for study purposes, not standard native writing.

Sometimes, but it changes the nuance.

• トライアル期間は短い。 = As for the trial period, it is short.
  This is a neutral statement about the topic.

• トライアル期間が短い。 = The trial period is what is short / It is the trial period that is short.
  This can sound more focused, contrastive, or context-dependent.

For a simple statement like The trial period is short, は is usually the most straightforward choice.

Grammatically, it is a compound noun made from two nouns:

• トライアル
• 期間

Together they function as one unit:

• トライアル期間 = trial period

Japanese creates many nouns this way, just by putting nouns together.

Examples of the same pattern:

• 試験期間 = exam period
• 契約期間 = contract period
• 使用期間 = usage period

So in this sentence, トライアル期間 is the topic marked by は.