Это фильм, о котором мой друг говорит.

Because the verb говорить here means to talk about, and in Russian that idea is expressed with the preposition о + the prepositional case.

So inside the relative clause, Russian is really saying:

• мой друг говорит о фильме = my friend is talking about the film

When фильме is replaced by a relative word, it becomes:

• о котором

So который changes form because it has to match the grammar required by говорить о.

Котором is the prepositional singular form of который.

We know this because:

1. the preposition о normally takes the prepositional case
2. the noun being referred to is фильм, which is:
   • masculine
   • singular

So the relative pronoun has to be masculine singular prepositional:

• который → basic form
• о котором → prepositional form after о

It does both, in different ways:

• It agrees with фильм in gender and number
• Its case is determined by its role inside the relative clause

So here:

• фильм is masculine singular
• inside the clause, the pronoun is the object of говорить о
• therefore we get о котором

This is a very important Russian pattern:
gender/number come from the noun being described, but case comes from the job the pronoun does in its own clause.

Because о котором мой друг говорит is a relative clause describing фильм.

Russian normally separates this kind of descriptive clause with a comma:

• Это фильм, о котором мой друг говорит.

This is similar to how English often sets off a relative clause, although Russian punctuation rules are not always identical to English ones.

Это is very common in sentences that identify or point something out:

• Это фильм... = This is the film... / It is the film...

It helps introduce the noun naturally.

Without это, you would usually not have a complete sentence here:

• Фильм, о котором мой друг говорит
  This feels like a phrase, not a finished statement.

So это makes the sentence complete and natural.

In the present tense, Russian usually does not use a word for is/am/are in sentences like this.

So:

• Это фильм literally looks like This film
• but it means This is a film / It is the film

This is normal Russian grammar.
In past and future tenses, forms of быть do appear, but not usually in simple present-tense identification sentences.

Russian has no articles like a and the.

So фильм can mean:

• a film
• the film

The exact meaning depends on context.

In this sentence, the context makes it clear that a specific film is meant, so English uses the, but Russian just says фильм.

Russian word order is more flexible than English word order.

In this sentence, putting говорит at the end is natural because the relative clause is built as:

• о котором мой друг говорит

This is a normal Russian pattern.

You may also hear:

• Это фильм, о котором говорит мой друг.

That is also correct. The difference is mostly one of emphasis and style, not basic meaning.

In Russian, prepositions stay directly with the word they govern.

So if English says:

• the film which my friend talks about

Russian does not leave the preposition stranded at the end. It keeps it in front:

• фильм, о котором мой друг говорит

This is one of the big differences from English relative clauses.

Yes, very often you can say:

• Это фильм, про который мой друг говорит.

This is common in spoken Russian.

The difference is roughly:

• о котором = more neutral, standard, a bit more formal
• про который = more conversational

Both are understandable and natural, but о котором is a very good form to learn because it fits standard grammar neatly with говорить о.

Because мой друг is already the subject.

Russian does not need an extra pronoun here:

• мой друг говорит = my friend is talking

Adding он would usually be unnecessary:

• мой друг, он говорит... would sound marked or unusual in this context

So the sentence is perfectly normal as it is.

Not in standard Russian here.

You cannot normally say:

• Это фильм, о что мой друг говорит.

That is incorrect.

For a relative clause after a noun like фильм, Russian normally uses forms of который:

• фильм, о котором...

The word что can introduce some kinds of clauses, but it does not work as a direct replacement for о котором in this sentence.

The dictionary form is говорить.

Говорит is the 3rd person singular present form:

• я говорю = I speak / I am speaking
• ты говоришь
• он/она говорит

Since the subject is мой друг (my friend), Russian uses the he/she/it form:

• мой друг говорит

So the ending matches the subject.