La farbo kaj la lako estas pretaj.

Because both nouns are definite. Esperanto often repeats la before each noun in a coordinated phrase when both are meant specifically.

So la farbo kaj la lako clearly means the paint and the lacquer.

You may sometimes see the article omitted before the second noun, but repeating it is the clearest and safest pattern for learners.

The -j marks the adjective as plural.

In Esperanto, adjectives agree with the nouns they describe in number and case. Here, the subject is la farbo kaj la lako, which is plural because it refers to two things together. So the adjective must also be plural:

• preta = ready, singular
• pretaj = ready, plural

That is why the sentence uses pretaj.

Esperanto verbs do not change for person or number.

So:

• mi estas = I am
• li estas = he is
• ili estas = they are

The same form, estas, works for all subjects. Unlike English, Esperanto does not switch between is and are.

Because there is no direct object here.

La farbo kaj la lako is the subject, and pretaj is a predicate adjective after the linking verb estas. Predicate adjectives do not take -n just because they come after the verb.

So this is simply:

• subject: la farbo kaj la lako
• verb: estas
• description: pretaj

No accusative is needed.

Each noun by itself is singular, but when two singular nouns are joined by kaj, together they form a plural idea.

So:

• la farbo estas preta = the paint is ready
• la lako estas preta = the lacquer is ready
• la farbo kaj la lako estas pretaj = the paint and the lacquer are ready

Two singular items joined by kaj behave as a plural subject.

Yes. That is also correct and very natural.

Pretas means roughly is/are ready. Esperanto often allows a root to take a verb ending directly, so:

• estas preta/pretaj
• pretas

Both work.

So these are both fine:

• La farbo kaj la lako estas pretaj.
• La farbo kaj la lako pretas.

The version with estas pretaj may feel a little more explicit to a beginner, but both are standard.

No, Esperanto word order is fairly flexible, though the basic neutral order is:

subject + verb + complement

So La farbo kaj la lako estas pretaj is the most neutral version.

But other orders are possible for emphasis, for example:

• Pretaj estas la farbo kaj la lako.

That sounds more marked or stylistic. For everyday use, the original order is the best one to learn first.

They are related materials, but they are not the same thing.

• farbo = paint
• lako = lacquer, varnish, or a similar finishing coat depending on context

So the sentence is talking about two different substances, not two words for the same thing.

Yes, that is grammatically possible, but the meaning changes.

Without la, the nouns are not specifically marked as the paint and the lacquer. The sentence becomes more general or less specifically identified.

So:

• La farbo kaj la lako estas pretaj = the specific paint and the specific lacquer are ready
• Farbo kaj lako estas pretaj = paint and lacquer are ready / paint and lacquer, in general or unspecified ones, are ready

If the intended meaning is definite, keep la.

Kaj is pronounced approximately like kai in English.

A useful pronunciation tip is that Esperanto j sounds like English y.

So:

• kaj sounds like kai
• pretaj ends with a sound like -tai

That same j sound appears in many plural endings and word endings in Esperanto.