Li metas la negativon apud la diapozitivon.

The -n marks the direct object.

In this sentence, li is the subject and metas is the verb, so the thing being placed is the direct object: la negativon.

A quick breakdown:

• li = subject
• metas = verb
• la negativon = direct object

In Esperanto, direct objects usually take -n.

This is a very common question, because prepositions normally do not require -n.

Here, the -n after apud shows direction or movement toward a position, not just a fixed location.

So:

• apud la diapozitivo = beside the slide, in a static sense
• apud la diapozitivon = to a position beside the slide

Since metas involves placing something somewhere, Esperanto can use apud + -n to show that movement toward that location.

So the sentence is not just saying where something is; it is saying where the negative is being moved/placed.

Apud means beside, next to, or by the side of.

It usually suggests fairly close physical nearness.

For example:

• La libro estas apud la lampo. = The book is beside the lamp.
• Li sidas apud mi. = He is sitting next to me.

In your sentence, it tells you the negative is being placed next to the slide.

La is the Esperanto definite article, meaning the.

It is used here because the sentence is talking about specific items:

• la negativo = the specific negative
• la diapozitivo = the specific slide

Esperanto has only one article: la. It does not have separate words for a or an.

So:

• la negativo = the negative
• negativo = a negative / negative in a general sense, depending on context

Because la is invariable. It never changes form.

Even when the noun takes -n, the article stays la:

• la negativo
• la negativon

The same thing happens with plural:

• la negativoj
• la negativojn

So the case ending goes on the noun, not on la.

Because -as is the Esperanto ending for the present tense.

So:

• metas = puts / is putting
• metis = put / was putting
• metos = will put

The root is met-, meaning put or place.

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command/wish
• -i = infinitive

Yes, Esperanto word order is fairly flexible, because endings like -n show what role a word has in the sentence.

The normal, neutral order here is:

• Li metas la negativon apud la diapozitivon.

But other orders are possible, for example:

• La negativon li metas apud la diapozitivon.
• Apud la diapozitivon li metas la negativon.

These alternatives may change the emphasis, but the -n helps you see which noun is the direct object.

Yes. They are standard noun forms in Esperanto.

Like many technical or international words, they look similar to words in European languages, but they follow normal Esperanto grammar:

• negativo = negative
• diapozitivo = slide / transparency

Because they are nouns, they end in -o. Then they take normal grammatical endings:

• negativo → negativon
• diapozitivo → diapozitivon

So even borrowed or international-looking words behave just like any other Esperanto noun.

Yes, but it would change the grammar or the nuance.

• Li metas la negativon apud la diapozitivo.
  This would still mean he places the negative beside the slide, but it presents apud la diapozitivo more as a location.

• Li metas la negativon apud la diapozitivon.
  This emphasizes movement to a place beside the slide.

So the first -n on negativon is required because it is the direct object.
The second -n on diapozitivon is optional in the sense that it expresses a specific idea: motion toward that position.

Yes. Li means he.

Esperanto personal pronouns include:

• mi = I
• vi = you
• li = he
• ŝi = she
• ĝi = it
• ni = we
• ili = they

So if the person were female, you would use ŝi:

• Ŝi metas la negativon apud la diapozitivon.