Cum ventus subito crescat, gubernaculum firmum esse maxime necesse est.

After cum, Latin often uses the subjunctive when the clause gives background circumstance, time, cause, or concession. Here cum ventus subito crescat means something like when or whenever the wind suddenly rises, not just the plain statement the wind rises.

So crescat is subjunctive because it is inside a cum-clause of this kind.

Crescat is:

• 3rd person singular
• present subjunctive
• active
• from crescere = to grow, increase, rise

Its subject is ventus.

No. Cum has two very common uses:

• as a preposition meaning with, followed by an ablative noun
• as a conjunction meaning when, since, or although, followed by a clause

Here it is clearly a conjunction, because it introduces a clause with its own subject and verb: ventus ... crescat.

All three are possible meanings of cum in different contexts, so this is a very common question.

In this sentence, the most natural sense is when or whenever. The clause gives the situation in which the main statement applies: when the wind suddenly rises, a firm rudder is especially necessary.

A translation with since could also work in some contexts, but although is less likely here.

Latin has no articles. That means there is no separate word for the or a/an.

So:

• ventus can mean wind, the wind, or a wind
• gubernaculum can mean rudder, the rudder, a rudder, helm, or the helm

Context tells you which English article is best.

Subito is an adverb meaning suddenly. It modifies crescat.

So:

• ventus subito crescat = the wind suddenly rises

It is not describing a noun, so it does not need to agree with anything.

Ventus is nominative singular. It is the subject of crescat.

So in the clause cum ventus subito crescat, ventus is the thing doing the rising.

The form gubernaculum can be either nominative singular or accusative singular, because it is a second-declension neuter noun, and those two cases are identical in the singular.

In this sentence, it is the noun tied to firmum esse. In sense, it is the thing that is to be firm.

For a learner, the important point is that gubernaculum and firmum belong together.

Because firmum is an adjective, not an adverb.

It describes gubernaculum, so it agrees with it:

• gubernaculum = neuter singular
• firmum = neuter singular

So gubernaculum firmum esse means for the rudder/helm to be firm.

If Latin wanted an adverb like firmly, that would be a different idea.

Because Latin often uses an infinitive phrase after impersonal expressions like necesse est.

Here:

• gubernaculum firmum esse = the rudder to be firm
• more natural English: that the rudder be firm

So esse is what lets Latin build that infinitive phrase.

Necesse est is an impersonal expression meaning it is necessary.

It does not usually have a personal subject like the boy is necessary. Instead, what is necessary is expressed by another word or phrase. Here that is the infinitive phrase:

• gubernaculum firmum esse maxime necesse est

Literally, this is something like:

• for the rudder to be firm is especially necessary

More natural English would be:

• it is especially necessary that the rudder be firm

Maxime is an adverb. Here it modifies necesse est.

So the meaning is:

• especially necessary
• or most necessary
• sometimes simply very necessary, depending on style

It is strengthening the idea of necessity, not describing the rudder itself.

A helpful breakdown is:

• Cum ventus subito crescat = subordinate cum-clause
• gubernaculum firmum esse = infinitive phrase
• maxime necesse est = main statement

So the sentence moves like this:

1. first, the situation: when the wind suddenly rises
2. then, the main point: it is especially necessary
3. then, what is necessary: for the rudder to be firm

Not in the same way it is in English. Latin word order is much more flexible because the endings show the grammar.

This sentence is perfectly normal Latin style:

• the cum-clause comes first
• the important idea necesse est comes near the end
• the infinitive phrase gubernaculum firmum esse sits before it

So you should rely more on endings and constructions than on English-style word order.

It can be translated either way, depending on context.

Its basic idea is the steering apparatus of a ship. In English, rudder may sound more literal and physical; helm may sound more idiomatic in some translations.

So if the meaning already given to you uses either one, that is probably fine.