Наша кафедра находится на третьем этаже старого корпуса, и на кафедре мне посоветовали раньше выбрать тему реферата.

Here кафедра means an academic department or chair at a university.

In this sentence it is used in two closely related ways:

• Наша кафедра находится... = Our department is located...
• на кафедре мне посоветовали... = at the department / in the department office, they advised me...

So yes, it is basically the same word both times, but in the second part it refers more to the people or office of the department.

Important: in other contexts кафедра can also mean a lectern, podium, or pulpit, but not here.

Находится means is located / is situated.

Russian often uses находиться when talking about where something is physically located, especially in a neutral or slightly formal style:

• Кафедра находится на третьем этаже. = The department is on the third floor.

Using есть here would not sound natural. Russian does not usually use есть the way English uses is for simple location.

A more conversational version could be:

• Наша кафедра на третьем этаже...

But находится is perfectly normal and a bit more explicit.

This is the prepositional case, because на is being used to show location:

• на этаже = on the floor

The ordinal number третий behaves like an adjective and agrees with этаж:

• nominative: третий этаж
• prepositional: на третьем этаже

So both words change:

• этаж → этаже
• третий → третьем

Because корпус depends on этаж.

The phrase literally works like:

• on the third floor of the old building

In Russian, that of the old building part is expressed with the genitive case:

• этаж чего? = floor of what?
• корпуса

So:

• на третьем этаже старого корпуса

Here:

• корпуса is genitive singular of корпус
• старого agrees with it in genitive singular

Also, at a university, корпус often means a building, block, or main building section, not body.

Because на + prepositional case is commonly used for being at or in certain places or institutions.

So:

• на кафедре = at the department / in the department office

The noun changes like this:

• nominative: кафедра
• prepositional: на кафедре

This is similar to other common patterns such as:

• на работе = at work
• на факультете = at the faculty
• на почте = at the post office

Because посоветовать takes the person receiving the advice in the dative case.

So:

• мне посоветовали = they advised me
• literally: to me, they advised

This is a very common pattern:

• Мне посоветовали прочитать эту книгу. = I was advised to read this book.
• Ему посоветовали отдохнуть. = He was advised to rest.

So мне is there because I am the recipient of the advice.

This is a very common Russian construction: 3rd person plural with no stated subject.

It means something like:

• they advised me
• someone advised me
• I was advised

The exact people are left unspecified, but from context we understand it means people at the department.

So:

• на кафедре мне посоветовали...

naturally means:

• At the department, they advised me...
• or more idiomatically in English, I was advised at the department...

Russian uses this structure a lot when the doer is unknown, unimportant, or obvious from context.

This is about aspect.

• выбрать = perfective → to choose, as a completed one-time action
• выбирать = imperfective → to be choosing, to choose repeatedly, to engage in the process of choosing

Here the advice is to complete one specific action:

• выбрать тему реферата = choose a topic for the paper

So выбрать is the natural choice.

If you used выбирать, it would sound more like focusing on the process or repeated action, which is not the main idea here.

Here раньше means earlier, sooner, or in advance.

It modifies выбрать:

• раньше выбрать тему = to choose the topic earlier / sooner

It does not necessarily mean before some explicitly mentioned event. It often just means earlier than one otherwise would or earlier than is usual.

Also, раньше is historically the comparative of рано:

• рано = early
• раньше = earlier

But in modern usage, you can simply understand it here as an adverb meaning earlier.

• тему is accusative singular, because it is the direct object of выбрать
• реферата is genitive singular, because it means of the paper / report

So the structure is:

• тема реферата = the topic of the paper
• выбрать тему реферата = to choose the topic of the paper

This is a standard Russian noun + genitive pattern.

Also, реферат in Russian academic usage usually means a short written academic paper, report, or essay-like assignment. It does not always match the English word referat, which is not commonly used in everyday English.

Yes, Russian could avoid the repetition, but repeating it is normal and clear.

For example, you might also see something like:

• Наша кафедра находится на третьем этаже старого корпуса, и там мне посоветовали...

But repeating на кафедре is often more precise than using там, especially in formal or neutral writing.

Russian frequently repeats nouns where English might prefer a pronoun or a vaguer word like there. So this repetition is not strange.

Because the two parts describe two different time frames:

• Наша кафедра находится... = a general present fact about location
• мне посоветовали... = a past event that happened to the speaker

This is completely normal. English does the same thing:

• Our department is on the third floor, and there they advised me...

So the sentence combines:

• a present-state description
• a past completed action

There is no tense problem here.