Februarius brevior est quam Ianuarius, sed mense Februario saepe ventus fortis manet.

Latin does not have articles. There is no direct equivalent of English the or a/an.

So:

• Februarius can simply mean February
• ventus fortis can mean a strong wind or the strong wind, depending on context

A reader understands from the sentence and situation whether the sense is definite or indefinite.

Brevior is the comparative form of brevis, which means short.

• brevis = short
• brevior = shorter

Since the sentence compares February with January, Latin needs the comparative: Februarius brevior est = February is shorter.

Also, brevior agrees with Februarius in:

• case: nominative
• number: singular
• gender: masculine

Quam means than in comparisons.

So:

• brevior ... quam Ianuarius = shorter than January

A very common Latin pattern is:

comparative adjective + quam + noun in the same case

Here:

• Februarius is nominative
• Ianuarius is also nominative after quam

That is why Latin says quam Ianuarius, not some other case.

Februarius is nominative because it is the subject of est.

Ianuarius is also nominative because after quam, Latin normally keeps the compared noun in the same case as the thing it is being compared with.

So the structure is:

• Februarius = subject, nominative
• Ianuarius = compared with the subject, so also nominative

This is very natural Latin in comparisons.

There is no separate word for in here because Latin often uses the ablative case by itself to express time when.

So:

• mense Februario = in February / in the month of February

This is called the ablative of time when.

English uses a preposition:

• in February

Latin often does not:

• Februario
• mense Februario

Because Februario agrees with mense.

• mense = ablative singular of mensis (month)
• Februario = ablative singular masculine, agreeing with mense

Literally, the phrase is something like:

• in the February month

Since the whole phrase is being used as an ablative expression of time, both words are ablative.

Yes, Latin often can simply say Februario to mean in February.

Adding mense makes the phrase fuller and a little more explicit:

• Februario = in February
• mense Februario = in the month of February

This is useful for style, clarity, or emphasis.

It also reflects the fact that month names were originally adjectival in expressions like mensis Februarius.

Because fortis belongs to a different adjective pattern.

Not all Latin adjectives use endings like -us, -a, -um.
Fortis, forte is a third-declension adjective.

Its nominative singular masculine/feminine form is fortis.

So:

• ventus = masculine singular nominative
• fortis = masculine singular nominative, agreeing with ventus

The important rule is that adjectives agree with nouns in:

• gender
• number
• case

They do not have to belong to the same declension.

Latin word order is much more flexible than English word order.

So both of these are possible:

• ventus fortis
• fortis ventus

Both mean strong wind.

The adjective after the noun is completely normal Latin. Word order often depends on:

• emphasis
• rhythm
• style

In this sentence, ventus fortis manet sounds natural and balanced.

Manet means remains, stays, or continues.

So ventus fortis manet does not just mean that a strong wind exists. It suggests that the strong wind continues or persists.

That gives the sentence a slightly stronger idea than est:

• ventus fortis est = there is a strong wind / the wind is strong
• ventus fortis manet = a strong wind remains / the strong wind continues

So manet adds the sense of duration or persistence.

Saepe means often, and Latin adverbs can move around fairly freely.

Here it comes before ventus fortis manet:

• saepe ventus fortis manet

That is a natural position, and it makes saepe modify the whole idea that follows: a strong wind often remains.

In Latin, adverb placement is less rigid than in English. The exact position can be chosen for:

• emphasis
• flow
• style

So the placement here is normal, not unusual.