Обов’язок складний.

In the present tense, Ukrainian usually leaves out the verb to be.

So:

• Обов’язок складний = The duty is difficult

Literally, it is more like:

• Duty difficult

This is completely normal in Ukrainian.
If you want to say the same thing in the past or future, forms of бути (to be) do appear:

• Обов’язок був складний = The duty was difficult
• Обов’язок буде складний = The duty will be difficult

Складний is an adjective.

In this sentence, it is not just describing a noun inside a noun phrase. It is acting as a predicate adjective:

• Обов’язок = the subject
• складний = what is being said about the subject

So the structure is:

• noun + adjective
• literally: Duty difficult
• naturally: The duty is difficult

The ending -ий shows that the adjective is:

• masculine
• singular
• nominative

It matches обов’язок, which is a masculine singular noun.

Ukrainian adjectives must agree with the noun they refer to in:

• gender
• number
• case

Compare:

• обов’язок складний — masculine singular
• робота складна — feminine singular
• завдання складне — neuter singular
• обов’язки складні — plural

A good clue is the dictionary form ending.

Обов’язок ends in a consonant, and nouns ending in a consonant are often masculine in Ukrainian.

So:

• обов’язок = masculine noun
• therefore the adjective must also be masculine:
  • складний

This is why you do not say Обов’язок складна or Обов’язок складне.

Here it is in the nominative case, because it is the subject of the sentence.

The basic pattern is:

• Обов’язок = subject, nominative
• складний = predicate adjective, also in nominative form

This is the normal form you would expect in a simple statement like X is Y.

Yes, Ukrainian word order is fairly flexible.

The most neutral order is:

• Обов’язок складний.

But you can also say:

• Складний обов’язок.

However, that usually sounds more like a noun phrase meaning:

• a difficult duty

rather than a full sentence meaning:

• The duty is difficult

If you want a full sentence, Обов’язок складний is the clearer default.

A helpful rough pronunciation is:

• обов’язок ≈ obo-VYA-zok
• складний ≈ SKLAD-nyi

The stress is usually shown as:

• обов’я́зок
• скла́дний

The apostrophe in обов’язок is important. It shows that the consonant before it stays hard and that the following я is pronounced like ya, not as a simple vowel that softens the previous consonant.

So в’я is pronounced roughly like vya.

It often means duty, but it can also mean obligation depending on context.

So обов’язок can refer to:

• a moral duty
• a responsibility
• an obligation

The exact English word depends on the situation.

For example:

• Це мій обов’язок = It is my duty / This is my responsibility
• Виконувати обов’язки = to carry out duties / fulfill responsibilities

Both can sometimes be translated as difficult or hard, but they are not always interchangeable.

• складний often means complex, difficult, complicated
• важкий often means heavy, and by extension hard, tough, difficult

So:

• Обов’язок складний suggests the duty is difficult/complex
• Обов’язок важкий suggests the duty is hard, burdensome, or tough

Sometimes either one works, but the nuance changes.

You usually would not say Це обов’язок складний in a basic neutral sentence.

Better options are:

• Обов’язок складний = The duty is difficult
• Це складний обов’язок = This is a difficult duty

The second sentence is different in structure:

• Це = this
• складний обов’язок = a difficult duty

So:

• Обов’язок складний = a statement about the duty
• Це складний обов’язок = identifying something as a difficult duty

You simply add не before the adjective:

• Обов’язок не складний. = The duty is not difficult.

This is the normal way to negate this kind of sentence in Ukrainian.

You could also say something stronger like:

• Обов’язок зовсім не складний. = The duty is not difficult at all.

Yes, it is a complete and natural sentence.

Short sentences like this are very common in Ukrainian, especially when the meaning is straightforward.

Ukrainian often allows compact statements such as:

• Дім великий. = The house is big.
• Завдання легке. = The task is easy.
• Обов’язок складний. = The duty is difficult.

So even though it looks short to an English speaker, it is fully grammatical and natural.