Si ventus deest, remigium nautis magno auxilio est.

Si means if. It introduces the condition:

• Si ventus deest = If the wind is lacking / if there is no wind

This is a very common Latin way to begin a conditional sentence.

Deest is the 3rd person singular present of deesse, which means to be lacking, to be absent, or to fail.

It is built from:

• de-
• est (from esse, to be)

So ventus deest means the wind is lacking or the wind fails.

Its dictionary forms are usually given as:

• desum, deesse, defui

Yes, deesse often takes a dative of the person or thing affected:

• mihi deest = it is lacking to me / I lack it

But it does not have to. It can also simply mean to be absent or to fail in a more general sense.

So here:

• ventus deest = the wind is absent / there is no wind

The sentence is not focusing on to whom the wind is lacking; it is just stating the condition.

Latin has no definite article (the) and no indefinite article (a/an).

So ventus can mean:

• wind
• a wind
• the wind

The context tells you which English wording sounds best. In this sentence, English naturally uses the wind or just wind.

Here remigium means rowing, the use of oars, or oar-power.

It does not mean a single oar. The idea is:

• when there is no wind,
• rowing is helpful to sailors.

So remigium is functioning as an abstract or collective noun.

Nautis is dative plural from nauta, nautae (sailor).

Here it means for the sailors or to the sailors.

So:

• nautis ... est = is ... for sailors

This is not possession. It is showing the people affected or benefited.

Magno auxilio is dative singular:

• magnus, -a, -um → magno
• auxilium, -i → auxilio

Together they mean something like:

• for great help
• more naturally in English: a great help

The adjective agrees with the noun:

• same case: dative
• same number: singular
• same gender: neuter

Yes. This is a very common Latin idiom.

• alicui auxilio esse = to be a help to someone
• magno auxilio esse = to be of great help

More specifically, nautis magno auxilio est is an example of the double dative construction:

• nautis = dative of reference/advantage (for sailors)
• magno auxilio = dative of purpose (for great help)

So the whole phrase means:

• it is of great help to sailors
• more natural English: it is a great help to sailors

The subject is remigium.

So the structure is:

• remigium = subject
• nautis = dative, to/for sailors
• magno auxilio = dative phrase, for great help
• est = is

In other words:

• remigium ... est = rowing is ...

Neither nautis nor auxilio is the subject, because both are in the dative.

Latin certainly could use a more direct verb such as adiuvare (to help). But Latin very often prefers noun-and-esse expressions like this one.

So instead of saying:

• rowing helps sailors

Latin says:

• rowing is for great help to sailors

This sounds less natural in literal English, but it is a normal and elegant Latin pattern.

Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

English depends heavily on position:

• Rowing helps sailors

Latin can move words around more freely because the cases already show their roles:

• remigium = subject
• nautis = dative plural
• magno auxilio = dative singular phrase

The sentence places nautis before magno auxilio est, which gives a smooth, natural Latin order. A Roman listener could still understand the grammar even if the order changed.

The present tense here expresses a general truth or regular situation:

• If wind is lacking, rowing is a great help to sailors.

It is not necessarily talking about one specific moment. Latin often uses the present tense this way, just as English does.