Li ne volas sukeron en la teo, sed li metas salon sur la ovon.

Because -n has an important grammatical job in Esperanto.

In this sentence, it does two different things:

• sukeron and salon are direct objects:
  • Li ne volas sukeron = He does not want sugar.
  • li metas salon = He puts salt.
• ovon is after the preposition sur, and here -n shows direction:
  • sur la ovon = onto the egg

So the same ending -n can mark either:

• the direct object, or
• movement/direction after a preposition.

The difference is:

• sur la ovo = on the egg as a location
• sur la ovon = onto the egg as a direction or destination

With metas (puts), movement is involved, so sur la ovon is very natural: the salt is being put onto the egg.

Because here en la teo describes a location or state, not movement.

The idea is not he puts sugar into the tea, but rather he does not want sugar in the tea. So en la teo means in the tea in a static sense.

If you were describing movement into the tea, you could use a directional form in an appropriate context, for example with a verb like metas:

• Li metas sukeron en la teon = He puts sugar into the tea.

But in your sentence, en la teo is the normal choice.

Because sugar and salt here are being talked about as substances in a general, uncounted way.

So:

• sukeron = sugar
• salon = salt

This is similar to English, where we usually say sugar and salt, not the sugar and the salt, unless we mean some specific sugar or salt already identified in the situation.

Because la means the, and here the sentence is talking about a specific tea and a specific egg.

So:

• en la teo = in the tea
• sur la ovon = onto the egg

This sounds like the tea and the egg that are already part of the situation, for example the ones he is eating or drinking.

Because Esperanto normally states the subject clearly in each clause.

So:

• Li ne volas sukeron en la teo
• sed li metas salon sur la ovon

Even though it is the same person, the second verb metas still normally needs its subject li.

This is the most standard and natural way to say it.

Because ne usually goes before the word or phrase it negates.

Here:

• Li ne volas = He does not want

That is the normal placement.

If ne were moved, the meaning or emphasis could change. Esperanto is often quite logical about this: ne tends to stand right before what it is denying.

Not completely. Esperanto word order is more flexible than English word order, because endings like -n help show grammatical roles.

The sentence as given is the neutral, straightforward order:

• subject + negation + verb + object + phrase
• then another clause with sed

You could change the order for emphasis, but the original version is the most natural one for a learner:

• Li ne volas sukeron en la teo, sed li metas salon sur la ovon.

A quick breakdown:

• li = he
• ne = not
• vol-as = wants
  
  • -as = present tense
• suker-o-n = sugar
  
  • -o = noun
  • -n = direct object
• en = in
• la = the
• te-o = tea
• sed = but
• met-as = puts
• sal-o-n = salt
• sur = on / onto
• ov-o-n = egg
  
  • here -n shows direction after sur

So this sentence is a good example of how much Esperanto shows through its endings.

Not in the most natural version of this sentence.

You can sometimes omit la when speaking very generally, but here the sentence sounds like it refers to a particular cup of tea and a particular egg. Because of that, la is the normal choice.

So:

• en la teo
• sur la ovon

sound much more natural here than:

• en teo
• sur ovo