У четвер урок складний.

У четвер means on Thursday.

Here четвер is in the accusative case, which is commonly used after у/в for days of the week when talking about when something happens.

A helpful detail: четвер is a masculine inanimate noun, and for that type of noun, the nominative and accusative forms are the same. So even though it is accusative here, it still looks like четвер.

Ukrainian often alternates у and в for smoother pronunciation. They are usually the same in meaning here.

So both of these can mean on Thursday:

• у четвер
• в четвер

In this sentence, у четвер is completely natural. Learners should think of this mostly as a sound/flow choice, not a meaning difference.

In Ukrainian, the verb to be is usually omitted in the present tense.

So:

• урок складний

literally looks like lesson difficult, but it means:

• the lesson is difficult

English requires is, but Ukrainian normally does not say it here.

Because in this sentence складний is a predicate adjective, not just a regular adjective inside a noun phrase.

Compare:

• складний урок = a difficult lesson
• урок складний = the lesson is difficult

So the sentence is not just naming a difficult lesson. It is making a statement about the lesson.

Because it agrees with урок, which is masculine singular.

Ukrainian adjectives change form to match the noun in:

• gender
• number
• often case

Here:

• урок = masculine singular
• складний = masculine singular form

For comparison:

• лекція складна = the lecture is difficult
• завдання складне = the task is difficult
• уроки складні = the lessons are difficult

It can mean either, depending on context.

Common possibilities include:

• lesson
• class
• class period

So in English, the best translation depends on the situation. In a school timetable context, class may sound natural. In a more general teaching context, lesson may sound better.

No. Ukrainian word order is fairly flexible, and changes in order often change emphasis more than basic meaning.

У четвер урок складний is a natural way to say:

• first, when: on Thursday
• then, what is being talked about: the lesson
• then, what is said about it: is difficult

If you change the order, the focus can shift. For example:

• У четвер складний урок more easily sounds like On Thursday, there is a difficult lesson or Thursday has a difficult lesson
• Урок у четвер складний puts more focus on the lesson on Thursday

So the original sentence is a good pattern for On Thursday, the lesson is difficult.

By itself, у четвер usually means on Thursday as a time reference in context, often a specific Thursday.

It does not normally mean every Thursday.

If you want every Thursday, Ukrainian commonly uses:

• щочетверга = every Thursday

So:

• У четвер урок складний = On Thursday, the lesson is difficult
• Щочетверга урок складний = Every Thursday, the lesson is difficult