Prilagodba tijekom karantene je teška.

Because tijekom is a preposition that requires the genitive case, and the genitive singular of karantena is karantene.

So:

• nominative: karantena
• genitive: karantene

That is why you get tijekom karantene.

Tijekom is a preposition meaning during. In standard Croatian, it is used with the genitive.

Examples:

• tijekom dana = during the day
• tijekom godine = during the year
• tijekom karantene = during the quarantine

So in this sentence, tijekom karantene is a prepositional phrase telling you when the adaptation is difficult.

Because teška is an adjective agreeing with prilagodba, which is a feminine singular noun.

In Croatian, adjectives must agree with the noun they describe in:

• gender
• number
• case

Here:

• prilagodba = feminine singular
• therefore teška = feminine singular

Compare:

• zadatak je težak = the task is difficult masculine
• prilagodba je teška = the adaptation is difficult feminine
• pitanje je teško = the question is difficult neuter

A very common clue is the ending -a in the dictionary form nominative singular. Many Croatian nouns ending in -a are feminine.

So:

• prilagodba is feminine
• that is why the adjective is teška, not težak or teško

Of course, there are some exceptions in Croatian, but -a = often feminine is a very useful basic rule.

Je is the 3rd person singular present tense of biti = to be.

The sentence structure is basically:

• Prilagodba = subject
• je = is
• teška = difficult

So je is necessary in a normal full sentence: Adaptation during quarantine is difficult.

Croatian sometimes omits forms of to be in very informal styles, headlines, notes, or poetry, but in a standard sentence like this, je should be there.

Yes. Croatian word order is more flexible than English word order, although not completely free.

The most neutral version is:

• Prilagodba tijekom karantene je teška.

But other orders are possible for emphasis, for example:

• Tijekom karantene prilagodba je teška.
• Teška je prilagodba tijekom karantene.

These alternatives can sound more marked or stylistic. For a learner, the original order is a good, natural default.

Sometimes yes, but the meaning is slightly different.

• tijekom karantene = during quarantine
  This focuses on the time period.

• u karanteni = in quarantine
  This focuses more on being in that state or situation.

So:

• Prilagodba tijekom karantene je teška = adaptation is difficult during the quarantine period
• Prilagodba u karanteni je teška = adaptation in quarantine is difficult

They are close, but not always identical in nuance.

Yes. Za vrijeme karantene also means during quarantine and is very natural Croatian.

Difference in tone:

• tijekom karantene sounds a bit more formal or compact
• za vrijeme karantene is also standard, but slightly more expanded

Both are correct in many contexts.

Because Croatian does not have articles like English a/an/the.

So prilagodba can mean:

• adaptation
• the adaptation
• an adaptation

The exact meaning depends on context.

This is very normal in Croatian, and learners from English often notice it quickly because English almost always needs an article.

It is a predicate adjective. That means it comes after the verb to be and describes the subject.

Structure:

• Prilagodba = subject
• je teška = is difficult

So teška is not an adverb here. It agrees with the subject as an adjective.

That is why you cannot use teško here if the subject is prilagodba.
You need the feminine form teška.

It is in the nominative singular because it is the subject of the sentence.

So the sentence contains:

• prilagodba = nominative singular
• tijekom karantene = prepositional phrase with genitive
• je teška = predicate

This is a useful pattern to recognize: subject in nominative + form of biti + adjective agreeing with the subject.