Fluctus tam alti sunt ut rete in aqua paene lateat.

Tam ... ut is a very common Latin pattern meaning so ... that.

So here:

• tam alti = so high
• ut rete in aqua paene lateat = that the net is almost hidden in the water

This is called a result clause: the waves are so high that a certain result happens.

Because it is inside a result clause introduced by ut after tam.

Latin regularly uses the subjunctive in clauses like this:

• tam ... ut ...
• ita ... ut ...
• sic ... ut ...

So lateat is not subjunctive because the action is doubtful; it is subjunctive because the grammar of a result clause requires it.

Here fluctus is nominative plural, because it is the subject of sunt.

A learner often notices that fluctus looks like a singular form too. That is because fluctus is a 4th-declension noun, and in writing the nominative singular and nominative plural can look the same if macrons are not shown.

So in this sentence we know it is plural because:

• the verb is sunt = they are
• the adjective is alti = high in the masculine plural

That makes fluctus clearly the waves, not the wave.

Because alti has to agree with fluctus.

Fluctus is:

• masculine
• plural
• nominative

So the adjective must also be:

• masculine
• plural
• nominative

That gives alti.

Rete is nominative singular, and it is the subject of lateat.

So the structure is:

• main clause: Fluctus tam alti sunt
• subordinate clause: ut rete in aqua paene lateat

Inside the ut clause, rete is the thing that is hidden.

A beginner may wonder whether rete is an object, but it is not. Lateat here means lies hidden / is hidden, so rete is the subject.

Because lateo is an active verb with a stative meaning: to lie hidden, to be concealed, to escape notice.

So although lateat is grammatically active, in English we often translate it with a passive-sounding expression such as:

• is hidden
• is almost hidden
• lies almost hidden

That is normal and idiomatic.

Because in with the ablative usually means in or on in a static location.

Here the net is not moving into the water; it is in the water. So Latin uses:

• in aqua = in the water

If there were movement into the water, Latin would normally use the accusative:

• in aquam = into the water

Paene means almost, and here it modifies lateat.

So:

• paene lateat = is almost hidden / almost lies hidden

It tells us the net is not completely out of sight, but nearly so.

Latin does not have definite or indefinite articles like English the or a/an.

So:

• fluctus can mean waves or the waves
• rete can mean a net or the net

The exact English translation depends on the context. In a sentence like this, English usually prefers the waves and the net.

Yes. Altus can mean either high or deep, depending on context.

For example:

• a mountain can be high
• water can be deep

With waves, English naturally says high, so tam alti fluctus means such high waves or waves so high.

Latin word order is more flexible because the endings show how words fit together.

English strongly prefers something like:

• The waves are so high that the net is almost hidden in the water.

Latin can arrange the words more freely:

• Fluctus tam alti sunt ut rete in aqua paene lateat.

Even though the order is different, the endings make the relationships clear:

• fluctus goes with alti
• fluctus is the subject of sunt
• rete is the subject of lateat
• in aqua gives the location

So the sentence is not random; it is just using normal Latin flexibility.

The subordinate clause begins with ut.

So the sentence divides like this:

• Fluctus tam alti sunt = main clause
• ut rete in aqua paene lateat = result clause

A useful habit is to spot markers like ut, because they often signal a new clause with its own verb. Here that verb is lateat.