Learn Hardcore Russian: Сегодня переписка с другом была длинной.

переписка is feminine singular.
In the past tense, Russian often uses the instrumental case for a descriptive predicate:
- была какой? длинной → adjective in instrumental feminine (-ой).

You could say:

Сегодня переписка с другом была длинная.

This uses nominative (длинная) and is also correct. The difference:

Instrumental (была длинной) often feels a bit more descriptive, like “turned out to be / was of the nature of being long”.
Nominative (была длинная) is more neutral, closer to a simple “was long”.

In everyday speech, both are fine; learners should just recognize that instrumental with быть in the past is very common.

Why is была used? I thought Russian often drops “to be”.

Russian drops быть (the verb to be) only in the present tense:

Переписка длинная. = “The correspondence is long.”
(no explicit есть)

But in the past (and future), you must use forms of быть:

Переписка была длинной. – “The correspondence was long.”
Переписка будет длинной. – “The correspondence will be long.”

So here, since this happened today but is already finished, Russian needs the past tense была.

Why is it с другом and not something like к другу or another case?

The preposition с + instrumental is the normal way to say “with (someone)”:

с кем? → с другом (with a friend)
(друг in instrumental is другом)

Some contrasts:

к другу (dative) = “to (toward) a friend” (direction).
у друга (genitive) = “at a friend’s / by a friend” (location/possession).

In your sentence, we mean “correspondence with a friend”, so с другом with the instrumental case is required.

What exactly does переписка mean here? Is it letters, texts, chat…?

Переписка is a general noun meaning “correspondence” or “exchange of written messages”. It can include:

Traditional letters.
Emails.
Messages in messengers / social networks.
SMS, chat messages, etc.

Context will specify the medium if needed:

переписка в чате – chat messages.
переписка по электронной почте – email correspondence.

In modern everyday speech, if you say переписка с другом, many people will imagine texting / online messages, unless context clearly suggests formal letters.

Why is переписка singular when in English we might say “our messages were long”?

Russian often uses an action noun in the singular to talk about the whole process:

переписка – the entire back‑and‑forth of writing.

So Сегодня переписка с другом была длинной focuses on the whole exchange being long, not on individual messages.

You could refer to individual items if you wanted:

Сегодня сообщения были длинные. – “The messages were long.”
Сегодня писем было много. – “There were many letters today.”

Why is была feminine? I would expect было or был.

The past tense of быть agrees in gender and number with the subject:

Он был дома. – He was at home.
Она была дома. – She was at home.
Оно было дома. – It was at home.
Они были дома. – They were at home.

Here, the subject is переписка, which is:

feminine,
singular.

So:

переписка была длинной – была must be feminine singular to match переписка.

Is there any difference between длинной and долгой in this sentence?

Both can be used, but they have different shades of meaning:

длинный / длинная / длинной – “long” mainly in a spatial or measurable sense (physically long, long text, long list, long story).
долгий / долгая / долгой – “long” mainly in a temporal sense (takes a long time, lasts long).

For переписка:

длинная переписка – could emphasize there were many messages, or messages were long (a lot of “text”).
долгая переписка – emphasizes that it took a long time.

In real usage, people might mix them a bit, but this is the core difference.
Your sentence with длинной is fine, but долгой would also be very natural: Сегодня переписка с другом была долгой.

Can I change the word order? For example: Переписка с другом сегодня была длинной?

Yes. Russian word order is fairly flexible. All of these are possible:

Сегодня переписка с другом была длинной.
Переписка с другом сегодня была длинной.
Переписка с другом была сегодня длинной.

Differences are mostly about what you emphasize:

At the beginning, Сегодня emphasizes “today” as the main time frame.
Starting with Переписка с другом highlights the correspondence itself first.

For a neutral, spoken style, your original Сегодня переписка с другом была длинной is very typical.

Could I say this in the present tense: Сегодня переписка с другом длинная?

You can, but it changes the meaning and sounds natural only if the correspondence is still ongoing.

Сегодня переписка с другом длинная.
→ “Today the correspondence with my friend is long (and is still going on).”

If you are talking after it has finished, you normally use the past:

Сегодня переписка с другом была длинной.

So, in most real-life situations where you are summarizing the day, the past tense is the right choice.

Is there a more natural way to say “Today I had a long correspondence with a friend”?

Your sentence is fine, but a very common everyday pattern in Russian is to use у меня был/была:

Сегодня у меня была длинная переписка с другом.

Literally: “Today by me was a long correspondence with a friend,” which corresponds to “I had a long correspondence with a friend today.”

Both:

Сегодня переписка с другом была длинной.
Сегодня у меня была длинная переписка с другом.

are correct; the second one makes “I had …” explicit, which often matches English more closely.