Mi ne plu estas laca.

In Mi ne plu estas laca, plu is part of the very common combination ne plu, which means no longer or not anymore.

So:

• ne plu = no longer
• Mi ne plu estas laca = I am no longer tired

On its own, plu often has the idea of further, more, or any more depending on context. But learners most often meet it first in ne plu.

Examples:

• Li ne plu loĝas ĉi tie. = He no longer lives here.
• Ĉu vi volas plu? = Do you want more? / Do you want to continue?
  (depending on context)

Esperanto often builds meanings with small, flexible words instead of using a single special word.

Here:

• ne = not
• plu = further / anymore / more
• ne plu = not anymore / no longer

This is completely normal Esperanto and one of the standard ways to express that something used to be true but is not true now.

Because ne plu works together as a unit meaning no longer, and estas is the verb.

So the sentence is structured like this:

• Mi = subject
• ne plu = negation + adverbial idea (no longer)
• estas = am
• laca = tired

Esperanto usually puts ne before what it negates, and ne plu very commonly comes before the verb:

• Mi ne plu laboras. = I no longer work.
• Ŝi ne plu kantas. = She no longer sings.

That makes Mi ne plu estas laca the most natural standard order.

It is much less natural. Standard Esperanto strongly prefers:

• Mi ne plu estas laca

Putting ne plu before the verb is the normal pattern. A learner should stick with that.

So even though Esperanto word order is somewhat flexible, this sentence is best learned as:

• Mi ne plu estas laca

Because Esperanto normally needs a verb in a full sentence, just like English needs am in I am tired.

So:

• Mi estas laca. = I am tired.
• Mi ne plu estas laca. = I am no longer tired.

Leaving out estas would usually sound incomplete.

Because laca is an adjective, and adjectives in Esperanto end in -a.

Here it describes mi:

• laca = tired

Since it is a predicate adjective after estas, it stays in the adjective form:

• Mi estas laca. = I am tired.

If the subject were plural, the adjective would also become plural:

• Ni ne plu estas lacaj. = We are no longer tired.

Because it is not a direct object.

The -n ending is usually for the direct object, but in this sentence laca is a predicate adjective after estas, so it does not take -n.

Compare:

• Mi vidas lacan homon. = I see a tired person.
  Here lacan gets -n because it describes the direct object homo.

But:

• Mi estas laca. = I am tired.
  No -n, because laca is linked to the subject through estas.

Usually laca means tired, weary, or fatigued.

If you specifically mean sleepy, Esperanto often uses:

• dormema = sleepy
• sometimes somnola = drowsy (less basic/common for beginners)

So:

• Mi estas laca. = I am tired.
• Mi estas dormema. = I am sleepy.

estas is the present tense of esti (to be).

So:

• Mi ne plu estas laca. = I am no longer tired.

Other tenses:

• Mi ne plu estis laca. = I was no longer tired.
• Mi ne plu estos laca. = I will no longer be tired.

This is very regular in Esperanto:

• estas = present
• estis = past
• estos = future

Yes, there is a small difference in emphasis.

• Mi ne plu estas laca = I am no longer tired
• Mi jam ne estas laca = I am already not tired / I am no longer tired

Both can often be translated similarly, but:

• ne plu emphasizes that a previous state has ended
• jam ne emphasizes that this change has already happened

So if you are learning the basic way to say no longer, ne plu is the safest and most direct choice.

Normally, no. Esperanto usually keeps the subject pronoun.

So you normally say:

• Mi ne plu estas laca.

Not just:

• Ne plu estas laca.

Without mi, the sentence would usually feel incomplete unless the subject is understood from a very special context.

Yes. It is completely natural and idiomatic Esperanto.

Very common ways to say this are:

• Mi ne plu estas laca.
• Mi jam ne estas laca.

The first one is especially useful to learn as the standard pattern for no longer:

• Mi ne plu estas laca.
• Li ne plu laboras ĉi tie.
• Ŝi ne plu estas malsana.