Pater dicit remigium in mari aspero saepe plus valere quam velum.

Because Latin commonly reports statements after verbs like dicit by using an accusative-and-infinitive construction rather than a word like that.

So:

• Pater dicit = Father says
• remigium ... valere = that rowing/oar-power is stronger / worth more / is more effective

In other words, Latin says something closer to Father says rowing to be more effective than a sail.

Because remigium is a neuter singular noun, and in Latin the nominative and accusative singular of neuter nouns are identical.

So in this sentence, remigium is functioning as the accusative subject of valere, even though its form looks the same as a nominative.

This is very common with neuter nouns:

• nominative singular: remigium
• accusative singular: remigium

Same spelling, different grammatical job.

Yes. In the comparison plus valere quam velum, the idea is:

• remigium ... plus valere
• quam velum = than a sail

More fully, you could understand it as:

• remigium plus valere quam velum [valere]

So velum is also accusative, as the understood subject of the omitted infinitive valere. Again, because it is neuter singular, its accusative form looks just like the nominative.

Because plus often means more in the sense of more in amount, force, or value. With valere, it is especially natural.

So plus valere means something like:

• to be stronger
• to have more force
• to count for more
• to be more effective

By contrast, magis is often used to modify adjectives, adverbs, or verbs in a different way, more like more in ordinary comparison. Here plus is idiomatic and very natural.

Valere basically means to be strong, to have power, or to be effective. Its meaning can shift a little depending on context.

Here it does not simply mean to be healthy. Instead, it means something like:

• to have more practical force
• to be more useful
• to work better
• to be more effective

So plus valere quam velum means that rowing or oar-power is more effective than a sail in those conditions.

In mari aspero is ablative, because in with the ablative usually means in, on, or under in a location or situation.

Here:

• mare, maris is a neuter 3rd-declension noun
• ablative singular: mari
• aspero is the ablative singular of asper, aspera, asperum, agreeing with mari

So:

• in mari aspero = in a rough sea / on a rough sea

Because the adjective must agree with mari in case, number, and gender.

Since mari is:

• ablative
• singular
• neuter

the adjective must also be:

• ablative
• singular
• neuter

So the correct form is aspero.

Latin usually does not need a separate word like that in this construction. English says:

• Father says that rowing is more effective than a sail

Latin says:

• Pater dicit remigium ... valere quam velum

The accusative + infinitive does the job that that-clause does in English.

Latin word order is much more flexible because the endings show how the words relate to each other.

This sentence puts the words in an order that feels natural in Latin:

• Pater first: the speaker/source
• dicit early: the main verb
• then the reported statement
• saepe placed near plus valere, which it modifies in sense

English depends heavily on word order for grammar, but Latin depends more on inflections. So Latin can move words around for emphasis, rhythm, or style without changing the basic meaning.

Saepe means often, and it modifies the idea of plus valere.

So the sense is:

• rowing often proves more effective than a sail in a rough sea

It does not just describe dicit. The father is not merely often saying it; rather, what he says is that this is often the case.

Not necessarily. Latin often uses the singular for a general idea.

So:

• remigium can mean rowing or oar-power in general
• velum can mean a sail, or sail-power, in general

This is similar to English when we say something like A sail is less useful in rough seas or Rowing is better than a sail. The singular can express a whole class or method.

It is best understood as rowing or oar-power here.

The noun remigium can refer to:

• the act of rowing
• the use or power of oars
• sometimes a body of rowers, depending on context

In this sentence, because it is being compared with velum as a means of propulsion, oar-power or rowing is the most natural sense.

Classical Latin has no definite or indefinite article.

So pater could mean:

• the father
• a father
• simply father

And velum could mean:

• a sail
• the sail
• sail-power, more generally

The context tells you which is most natural. English must choose an article; Latin usually does not.