Learn Hardcore Russian: Например, сегодня я буду учителем, а мой друг — студентом. - For example, today I will be the teacher, and my friend will be the student.

студент → студентом
Final consonant т is “hard”, so you get -ом.
учитель → учителем
The stem ends in a soft consonant (ль), so you get -ем.

In short:

After most hard consonants: -ом (стол → столом)
After most soft consonants and й: -ем (герой → героем, учитель → учителем)

Why do we use the instrumental case after буду, but in other sentences I see just я учитель without any ending?

Russian can express “X is Y” in two main ways:

Nominative predicate (simple statement of identity)
- Я учитель. – I am a teacher.
  Neutral, permanent, or general fact.
Instrumental predicate (role, capacity, or a bit more “situational”)
- Я буду учителем. – I will be (serve as / act as) a teacher (today / in that context).

In your sentence, сегодня я буду учителем implies “Today I’ll take the role of teacher,” not necessarily a permanent occupation. That’s why instrumental is used.

Both patterns are correct; the choice slightly changes the nuance:

Он врач. – He is (by profession) a doctor.
Он был врачом. – He worked as / served as a doctor (role or past situation).

Why is there a dash (—) before студентом? Why not another будет?

In Russian, when linking two nouns (or noun-like words) in the present or future, you can omit the verb быть and instead use a dash:

Мой друг — студентом.
Here, the full form would be Мой друг будет студентом, but будет is omitted and replaced with a dash.

This is a normal stylistic option, especially in short, balanced phrases like:

Я буду учителем, а он — студентом.

Key points:

The dash stands in for the missing verb.
You cannot use a comma instead of the dash here.
Both мой друг будет студентом and мой друг — студентом (in this contrastive structure) are acceptable, with the dashed version sounding more compact and stylistically neat.

What is the difference in meaning between я буду учителем, а мой друг — студентом and я буду учителем, а мой друг будет студентом?

The meaning is essentially the same: I will be the teacher, and my friend will be the student.

The difference is stylistic/rhythmic:

…буду учителем, а мой друг будет студентом
– fully explicit, slightly heavier, more formal or neutral.
…буду учителем, а мой друг — студентом
– more compact and elegant, using the dash to avoid repetition.

Both are grammatically correct. The second is very typical in written Russian.

What does Например do here, and where can it be placed in the sentence?

Например means “for example” and introduces an example of a situation.

In your sentence:

Например, сегодня я буду учителем, а мой друг — студентом.
→ For example, today I’ll be the teacher and my friend (will be) the student.

You can move например around a bit:

Сегодня, например, я буду учителем…
Я, например, буду учителем, а мой друг — студентом.

All are possible; moving например changes which part feels like the “example” or emphasis, but the core meaning stays.

What exactly does а mean in а мой друг — студентом? Is it “and” or “but”?

А is a conjunction that often shows contrast or difference between two parts. It can be translated as:

“and” (but with a contrasting feel)
“whereas”
“while”

In your sentence:

…я буду учителем, а мой друг — студентом.
→ …I’ll be the teacher, while my friend will be the student.

It’s not a simple additive “and” (и), but more like “and on the other hand / as for my friend”.

Is the word order сегодня я буду учителем fixed, or can I say я сегодня буду учителем?

Word order in Russian is flexible, with differences in emphasis more than grammar.

All of these are possible:

Сегодня я буду учителем.
Я сегодня буду учителем.
Я буду сегодня учителем.

Nuances are small:

Сегодня я буду учителем – emphasizes today.
Я сегодня буду учителем – more neutral, like ordinary conversation.
Я буду сегодня учителем – slightly highlights the verb phrase “will be today”.

They all mean essentially Today I will be the teacher.

How is буду formed, and how do other forms of быть look in the future?

Буду is the 1st person singular future of быть (to be).

Future tense of быть:

я буду – I will be
ты будешь – you will be (singular, informal)
он / она / оно будет – he / she / it will be
мы будем – we will be
вы будете – you will be (plural / formal)
они будут – they will be

In your sentence:

я буду учителем – I will be (the) teacher.

Could I say я стану учителем instead of я буду учителем? What’s the difference?

Yes, but the meaning shifts:

Я буду учителем.
Often: “I will be (serve as) a teacher (in that situation / then).”
In your specific sentence, it sounds like a role for today.
Я стану учителем.
Means “I will become a teacher” (a change of status or profession in the future, after some process).

So стану emphasizes becoming, while буду emphasizes being.

Why is the case of учителем and студентом the same for both я and мой друг, even though they are different people?

The case of the predicate noun (учителем, студентом) is determined by its function, not by the specific subject:

After быть (буду / будет / etc.) expressing role or profession, the predicate usually appears in instrumental:
- Я буду учителем.
- Мой друг будет студентом.
- Она была врачом.
- Мы стали друзьями.

The instrumental form doesn’t change with the person of the subject; it changes only with number and gender of the predicate word itself:

singular masculine: учителем, студентом
singular feminine: учительницей, студенткой
plural: учителями, студентами

Is there any article hidden in учителем or студентом? How do I know if it’s “a teacher” or “the teacher”?

Russian has no articles (no “a/an” or “the”). Words like учителем and студентом can correspond to:

a teacher / the teacher
a student / the student

Context decides:

Сегодня я буду учителем, а мой друг — студентом.
In English, the most natural is Today I’ll be *the teacher, and my friend (will be) the student*, because we’re talking about specific roles in a specific situation.

But grammatically in Russian, nothing marks definiteness or indefiniteness; it’s all in context and sometimes intonation.

Why is there a comma after Например?

Например is used here as a sentence adverb/introduction (“for example”), so it’s treated like a separate introductory element. In Russian punctuation, such elements are usually set off by commas:

Например, сегодня я буду учителем…
К счастью, он уже пришёл. – Fortunately, he has already come.

You will almost always see a comma right after например when it starts the sentence.