Cum ventus subito crescit, nauta naviculam ad ripam ligat.

Because cum can be either:

• a preposition meaning with when it is followed by a noun in the ablative, or
• a conjunction meaning when, since, or although when it introduces a clause with a finite verb.

Here it introduces the clause cum ventus subito crescit, so it is a conjunction meaning when.

Crescit is the present tense of crescere, meaning grows, increases, or rises.

Latin often uses the present tense in sentences like this for:

• a general truth or repeated situation: whenever the wind suddenly rises
• a vivid present in narration: when the wind suddenly rises

So the present tense is completely natural here.

Ventus is nominative singular.

It is nominative because it is the subject of crescit. The wind is the thing doing the rising/increasing.

So:

• ventus = the wind
• crescit = rises / increases

Because nauta is a first-declension masculine noun.

Many beginners expect a noun ending in -a to be feminine, and often that is true, but some first-declension nouns referring to male people are masculine, such as:

• nauta = sailor
• poeta = poet
• agricola = farmer

Here nauta is nominative singular and is the subject of ligat.

Because naviculam is the accusative singular, used for the direct object.

The sailor is tying the little boat, so naviculam is the thing directly acted upon by ligat.

• nominative: navicula = the little boat
• accusative: naviculam = the little boat, as object

Yes. Navicula is a diminutive form.

• navis = ship, boat
• navicula = little boat, small boat

A diminutive can suggest small size, but sometimes it is just the ordinary word for a smaller kind of vessel, without much emotional tone.

Because ad takes the accusative case, and ripam is the accusative singular of ripa.

Ad usually means:

• to
• toward
• up to
• sometimes against

In this sentence, ad ripam ligat means the sailor ties the boat to the bank/shore.

Latin commonly uses ligare with a phrase showing the place or object to which something is tied, and ad + accusative is a very natural way to express that.

So:

• naviculam ligat = he ties the little boat
• ad ripam ligat = he ties it to the bank/shore

A learner should treat ligare ad + accusative as a normal and useful pattern.

Subito is an adverb, meaning suddenly.

It modifies crescit, telling us how the wind rises:

• ventus crescit = the wind rises
• ventus subito crescit = the wind suddenly rises

It is not describing a noun; it is describing the action.

The clause introduced by cum is:

Cum ventus subito crescit

That whole part is the subordinate clause. Then the main clause is:

nauta naviculam ad ripam ligat

A good clue is that each clause has its own finite verb:

• crescit in the cum clause
• ligat in the main clause

The word order is important for style and emphasis, but not as much for basic grammar as in English.

Latin relies heavily on endings, so the sentence could be rearranged more freely than an English sentence, as long as the forms stay clear.

The given order is very natural:

• the when clause comes first
• the main verb ligat comes at the end, which is also very common in Latin prose

Yes.

With cum, Latin can use:

• indicative for a straightforward temporal meaning, as here: when
• subjunctive for other shades of meaning, especially circumstantial, causal, or concessive uses

In this sentence, cum ... crescit is just a simple temporal clause, so the indicative is exactly what you would expect.

Ligat comes from ligare, meaning to tie, bind, or fasten.

In this context, the natural English idea is often ties up or moors the boat. So the sailor fastens the little boat to the bank.

That is a good example of how a basic Latin verb may need a slightly more idiomatic English translation depending on context.