Mi faras tion, kio estas necesa.

Because tion is the direct object of faras.

In Esperanto, direct objects take the -n ending. So:

• tio = that, that thing
• tion = that, that thing as the object

Here, Mi faras tion means I do that or I do that thing, so tion needs -n.

They do different jobs.

• tion points to something: that
• kio introduces and identifies what that thing is: what

So the structure is roughly:

• Mi faras tion = I do that
• kio estas necesa = which is necessary / what is necessary

Together, the sentence means something like: I do that which is necessary.

This pattern is common in Esperanto: a pointing word such as tio is followed by a ki- word that explains it.

Because kio is used for what, for a thing that is not being treated as a specific individual item.

A helpful contrast is:

• kio = what, what thing, that which
• kiu = which one, who

In this sentence, the idea is not which specific item, but rather what is necessary in a general sense. So kio is the natural choice.

Literally, it means what is necessary or that which is necessary.

It is a clause describing tion. So the whole sentence is saying:

I do that — the thing that is necessary.

In natural English, this is often expressed as I do what is necessary.

The comma separates the main clause from the following subordinate clause.

• Main clause: Mi faras tion
• Subordinate clause: kio estas necesa

Esperanto uses commas quite regularly before subordinate clauses, often more consistently than English does. So the comma here is normal and expected.

Because necesa is not a direct object here. It is a predicate adjective linked to kio by estas.

In kio estas necesa:

• kio is the subject of estas
• necesa describes kio

Predicate adjectives normally do not take -n unless there is some special reason unrelated to this basic pattern.

So:

• kio estas necesa = what is necessary

Faras is present tense: do / am doing.

So:

• Mi faras = I do / I am doing

Yes, you could change the tense if needed:

• Mi faris tion, kio estis necesa. = I did what was necessary.
• Mi faros tion, kio estos necesa. = I will do what will be necessary.

The original sentence is in the present tense throughout.

No, they mean different things.

• Mi faras tion, kio estas necesa. = I do what is necessary.
• Mi faras tion necesan. = I make that necessary.

In the second sentence, necesan describes tion as a result of the action, almost like I render that necessary. That is a different idea.

So the comma and the clause kio estas necesa are important.

Yes. That is a very natural alternative.

• kio estas necesa = what is necessary
• kio necesas = what is needed / what is necessary

The second version is a bit more compact because necesi can function as a verb. Both express nearly the same idea in this context.

Usually, Esperanto prefers Mi faras tion, kio estas necesa or simply Mi faras tion necesan only if the meaning is different.

Leaving out tion in this sentence would sound less standard for this exact structure, because tion ... kio ... is a common correlated pattern: that ... which ...

So for a learner, the safest version is:

Mi faras tion, kio estas necesa.