Learn Hardcore Russian: В нашей группе семьдесят студентов.

This is grammatically correct, but есть adds a slight nuance of “there exists / there really are,” and is more common when you want to emphasize existence, contrast, or new information. In neutral statements like this one it’s very often left out.

What case are нашей and группе, and why do they have those endings?

Both нашей and группе are in the prepositional case.

The preposition в
- a location (where?) requires the prepositional case.
  - в (чём?) → в нашей группе (“in our group”).

Declension:

группа (fem. noun)
- nominative: группа
- prepositional: (о) группе
наш (possessive pronoun “our”) in the feminine singular
- nominative: наша
- prepositional: (о) нашей

So в нашей группе = preposition в + prepositional нашей + prepositional группе. They agree in gender (feminine), number (singular), and case (prepositional).

Why can’t I say В наша группа семьдесят студентов?

Because after the preposition в (“in”) you must use the prepositional case, not the nominative.

наша группа = nominative (“our group” as subject).
After в (meaning “in”) you need в нашей группе (“in our group”).

So:

✗ В наша группа семьдесят студентов. — incorrect (nominative after в)
✓ В нашей группе семьдесят студентов. — correct (prepositional after в)

The preposition forces the case change.

What does группа mean here? Is it just “group” in general?

In everyday Russian, группа can mean a general “group” of people or things.

In an educational context, especially at universities and colleges, группа often means an assigned class / cohort / group of students who attend most classes together (similar to a “class section” or “homeroom group”).

So В нашей группе семьдесят студентов most naturally means:
“In our (academic) group / class there are seventy students.”

Why is it семьдесят студентов and not семьдесят студенты?

In Russian, numbers affect the case of the noun that follows. For most nouns:

After 5, 6, 7, …, 20, 30, 40, 50, … etc., the noun goes into the genitive plural.

So:

семьдесят (70) → noun must be genitive plural
студент (student)
- nominative plural: студенты
- genitive plural: студентов

Therefore you say:

✓ семьдесят студентов (seventy students)
✗ семьдесят студенты — wrong case (nominative instead of genitive).

The whole phrase семьдесят студентов is the subject, but inside that phrase the numeral governs the case of the noun.

What case is студентов, and why is that case used with the number семьдесят?

Студентов is in the genitive plural.

Rule (simplified) for cardinal numbers in the nominative:

1 → noun in nominative singular
- один студент
2–4 (except 12–14) → noun in genitive singular
- два студента, три студента
5–20, 25–30, 35–40, etc. → noun in genitive plural
- пять студентов, семьдесят студентов

Since семьдесят is ≥ 5 and ends in 0, the noun must be in genitive plural: студентов.

What gender is семьдесят? Does it agree with студентов?

The numeral семьдесят itself does not change for gender in the nominative/accusative and does not “agree” with the noun the way adjectives do.

You use семьдесят with any noun in the nominative:
- семьдесят студентов (masc.)
- семьдесят девушек (fem.)
- семьдесят писем (neut.)

In other cases it declines as a number, not for gender:

gen./dat./prep./instr. → семидесяти
- у семидесяти студентов (gen.) — “(from) seventy students”

So you don’t match семьдесят to the gender of студентов; you only put студентов in the correct case (genitive plural).

How do you pronounce семьдесят and where is the stress?

Семьдесят is pronounced approximately as [SYEM-dye-syat], with the stress on the first syllable:

СЕ́мь-де-сят

Details:

семь loses the clear [mʲ] + [ʲ] before д, so it’s more like “семд‑”.
десят is unstressed here, so е/я are shorter and less distinct.

So you get something close to [SYEM-dye-syat], not [syem-dye-SYAT].

Can I change the word order to Семьдесят студентов в нашей группе?

Yes, this is grammatically correct.

В нашей группе семьдесят студентов.
- Neutral: topic = “our group”; new info = “there are seventy students.”
Семьдесят студентов в нашей группе.
- Emphasis shifts slightly to the number: topic = “seventy students”; comment = “they are in our group.”

In many contexts the two sentences are effectively interchangeable; the difference is mainly in what you emphasize or present as known information.

Could I say В нашей группе есть семьдесят студентов? What’s the difference?

Yes, you can. Both sentences are correct:

В нашей группе семьдесят студентов.
В нашей группе есть семьдесят студентов.

The version with есть more strongly emphasizes existence / presence, often when:

you introduce this information as new or surprising, or
you contrast with something else (e.g., “В вашей группе двадцать, а в нашей группе есть семьдесят студентов”).

In many neutral situations, есть is simply omitted, and the shorter form sounds more natural.

Is it okay to write 70 instead of семьдесят in this sentence?

Yes. In everyday writing you can write either:

В нашей группе семьдесят студентов.
В нашей группе 70 студентов.

Both are acceptable.

In more formal prose (literature, some academic texts), numbers up to a certain size are often written out in words, but in normal practical usage digits are very common. The grammar of the sentence does not change; only the way the number is written.