Февраль был коротким, но очень холодным.

Был is the past-tense form of быть (to be).

It agrees with Февраль, which is:

• singular
• masculine

So:

• он был = he/it was
• она была = she/it was
• оно было = it was
• они были = they were

Because Февраль is a masculine singular noun, the sentence uses был.

Because after быть in the past tense, full-form predicate adjectives are very often put in the instrumental case.

So:

• короткий → коротким
• холодный → холодным

That is why the sentence says:

• Февраль был коротким
• not normally Февраль был короткий in neutral standard Russian

This is a very common pattern:

• День был длинным. = The day was long.
• Город был тихим. = The city was quiet.

They are both masculine singular instrumental, but they come from different adjective types.

• короткий → коротким
• холодный → холодным

So the difference comes from the adjective’s dictionary form and stem pattern, not from a difference in meaning or grammar.

For a learner, the main thing is to memorize the change from the base form:

• -кий adjectives often become -ким
• -ный / -ый adjectives often become -ным / -ым

Yes, that is grammatically possible.

Those are short-form adjectives:

• короток
• холоден

This version sounds more:

• literary
• formal
• bookish

The original sentence with full adjectives in the instrumental:

• Февраль был коротким, но очень холодным

is more natural for everyday neutral Russian.

Because both adjectives describe the same subject, Февраль, and both belong to the same predicate.

So Russian does not need to repeat the verb:

• Февраль был коротким, но очень холодным.

This works just like English:

• February was short but very cold.

You could repeat был, but it would usually sound unnecessary:

• Февраль был коротким, но был очень холодным.

That is grammatically possible, but less smooth here.

Because но means but, and in Russian it normally introduces a new clause or contrast, so it is preceded by a comma.

So:

• Февраль был коротким, но очень холодным.

This is standard punctuation.

Because очень means very, and it directly modifies the adjective холодным.

Russian usually places очень right before the word it intensifies:

• очень холодным = very cold
• очень длинным = very long
• очень интересным = very interesting

So the position is normal and expected.

No. In Russian, month names are normally written with a lowercase letter:

• январь
• февраль
• март

Here Февраль is capitalized only because it is the first word of the sentence.

If it were in the middle of a sentence, it would usually be:

• февраль

Yes.

Февраль is a masculine noun, even though it ends in -ь. In Russian, nouns ending in -ь can be masculine or feminine, so you have to learn the gender individually.

Here we know it is masculine because Russian month names like январь, февраль, апрель are masculine.

That is why the sentence has:

• был
• коротким
• холодным

all in masculine singular agreement.

Usually it refers to a particular February in context, for example:

• last year’s February
• this year’s February
• the February being discussed

Russian, like English, can use a month name this way when the context makes it clear.

So the sentence most naturally means something like:

• That February was short but very cold.

without needing an article.