Ego aquam vino praefero.

Yes. The -o ending in praefero already tells you the subject is I.

So ego is not necessary for basic grammar. It is added for emphasis, contrast, or clarity. In this sentence, ego can suggest something like I prefer water to wine, perhaps as opposed to someone else.

With praefero, the thing that is being preferred takes the accusative.

So in Ego aquam vino praefero, aquam means water as the thing I prefer.

A useful pattern is:

aliquid alicui praeferre = to prefer something to something

So:

• aquam = the thing preferred
• vino = the thing it is preferred to

Because praefero does not take two accusatives here. The second noun is normally in the dative.

So:

• aquam = accusative
• vino = dative singular

That gives the sense I prefer water to wine.

If you used vinum, that would not fit the normal construction of praefero in this sentence.

Formally, vino could be either dative singular or ablative singular, because those forms look the same in the second declension.

But in this sentence, it is understood as dative, because praefero commonly takes:

• an accusative for the thing preferred
• a dative for the thing compared with it

So the syntax shows that vino is functioning as dative here.

Praefero literally means something like I carry before or I place before:

• prae- = before
• fero = I carry

From that literal idea comes the common meaning I prefer.

So Latin is expressing preference through an image like I place water before wine.

No. It is built from fero, which is an irregular verb.

Its principal parts are:

praefero, praeferre, praetuli, praelatum

Because it comes from fero, some of its forms are less predictable than those of a regular verb.

Yes, Latin word order is more flexible than English word order because the endings show each word’s function.

So these can all mean essentially the same thing:

• Ego aquam vino praefero
• Aquam vino praefero
• Vino aquam praefero
• Aquam ego vino praefero

The choice of order can affect emphasis:

• putting ego first can emphasize I
• putting aquam first can emphasize water
• putting vino first can emphasize the contrast with wine

Latin has no articles like English the and a/an.

So aquam can mean:

• water
• the water
• sometimes some water

and vino can mean:

• wine
• the wine

You understand the exact sense from context.

Usually it means water in a general sense, especially in a sentence like this about preference.

So Ego aquam vino praefero most naturally means I prefer water to wine, not necessarily a particular glass of water over a particular glass of wine.

But Latin itself does not force that distinction here; context would decide.

Yes. Aquam vino praefero is a perfectly complete sentence.

That is often the more neutral Latin way to say it. Adding ego makes the subject more explicit and often more emphatic.

You can break the nouns down like this:

• aqua = nominative singular, water
• aquam = accusative singular
• vinum = nominative or accusative singular, wine
• vino = dative or ablative singular

In this sentence:

• aquam is accusative because it is the thing preferred
• vino is dative because it is the thing to which something is preferred

Not in good English, but that literal idea helps explain the Latin.

A more literal unpacking would be: I place water before wine
or
I carry water before wine

That is why the natural English translation is I prefer water to wine.