На филологическом факультете сегодня проходит интересная лекция, о которой все давно говорят.

In Russian, на is commonly used with факультет when you mean at the faculty / in the faculty division of a university.

• в университете = at the university
• на филологическом факультете = at the philology faculty

This is just the normal idiomatic choice in Russian. It does not literally mean on in this context.

Because на here expresses location, it requires the prepositional case, which is why you get:

• на факультете
• на филологическом факультете

Because after на in the meaning of location, Russian uses the prepositional case.

The noun факультет becomes:

• nominative: факультет
• prepositional: факультете

The adjective филологический has to agree with the noun in gender, number, and case, so it also changes:

• nominative masculine: филологический
• prepositional masculine: филологическом

So:

• на филологическом факультете = at the philology faculty

Here проходит means takes place, is being held, or is going on.

So:

• сегодня проходит интересная лекция = an interesting lecture is taking place today

This is a very common use of проходить with events:

• проходит концерт = a concert is taking place
• проходит выставка = an exhibition is being held
• проходит лекция = a lecture is taking place

So you should not translate it literally as passes here.

Because лекция is the subject of the sentence.

Even though it comes after the verb, it is still the thing that is taking place:

• проходит лекция = a lecture is taking place

Russian word order is flexible, so the subject does not always have to come first.

Yes, Russian word order is much more flexible than English word order.

This sentence begins with:

• На филологическом факультете сегодня...

That puts the setting first: at the philology faculty today...

Then comes the verb and subject:

• проходит интересная лекция

A different order is possible, for example:

• Сегодня на филологическом факультете проходит интересная лекция...
• Интересная лекция сегодня проходит на филологическом факультете...

These versions would still be grammatical, but they shift the emphasis slightly. The original sounds natural and gives the location first.

О которой means about which or more naturally in English, which ... about in a relative clause.

It refers back to лекция:

• лекция, о которой все давно говорят
• the lecture that everyone has been talking about for a long time

Russian often keeps the preposition together with the relative pronoun:

• о которой = about which

English usually moves the preposition to the end:

• the lecture which everyone is talking about

Because the pronoun has to match лекция and also take the case required by the preposition.

1. It refers to лекция, which is:

   • feminine
   • singular

2. The preposition о requires the prepositional case

So the form is:

• nominative feminine singular: которая
• prepositional feminine singular: которой

That is why we get:

• о которой

The preposition о means about and normally takes the prepositional case.

Examples:

• о лекции = about the lecture
• о книге = about the book
• о которой = about which

So in this sentence:

• о которой все давно говорят = about which everyone has long been talking

Here все means everyone / everybody.

So:

• все давно говорят = everyone has been talking for a long time

When все refers to people, it often works like everyone. The verb is plural:

• все говорят = everyone is talking / all are talking

This is normal in Russian.

Not exactly. Here давно means for a long time already or for a long time now, not simply long ago.

So:

• все давно говорят does not mean everyone talked about it long ago and stopped
• it means everyone has been talking about it for a long time

This is a very important difference. With a present-tense verb, давно often gives the idea of an action that started in the past and is still relevant now.

Because о которой все давно говорят is a relative clause describing лекция.

Russian separates this kind of clause with a comma:

• лекция, о которой все давно говорят

Just like in English, this part adds information about the lecture.

Yes, in many contexts Russian can also say:

• идёт лекция = a lecture is going on
• проходит лекция = a lecture is taking place / is being held

The difference is small:

• идёт often focuses on the lecture as something currently in progress
• проходит often sounds a bit more like is taking place / is being held

In this sentence, проходит is very natural.

In Russian, adjectives usually come before the noun they describe, just as in English:

• интересная лекция = an interesting lecture

The adjective agrees with the noun:

• лекция is feminine singular nominative
• so the adjective is интересная

That is why it is not интересный or интересное, but интересная.