Se ĉi tiu plumo ne funkcias, mi uzos alian plumon.

Se means if. It introduces a condition:

• Se ĉi tiu plumo ne funkcias = If this pen doesn't work

So the whole sentence has the structure:

• condition
  • result
• If this pen doesn’t work, I will use another pen.

In Esperanto, tiu means that one, and ĉi tiu means this one.

So:

• tiu plumo = that pen
• ĉi tiu plumo = this pen

The little word ĉi adds the idea of nearness, giving this instead of that.

You may also see the order tiu ĉi plumo. That means the same thing. Both are correct:

• ĉi tiu plumo
• tiu ĉi plumo

Because after if, Esperanto often works like English: it uses the present tense for a real future possibility.

So:

• Se ĉi tiu plumo ne funkcias literally looks like If this pen does not work
• but in context it means If this pen doesn’t work / If this pen won’t work

This is normal. The result clause shows the future clearly:

• mi uzos = I will use

Uzos is the future tense of uzi (to use).

Esperanto verb endings are very regular:

• uzi = to use
• uzas = use / am using
• uzis = used
• uzos = will use
• uzus = would use
• uzu = use! / let ... use

So mi uzos means I will use.

Because alian plumon is the direct object of uzos.

In Esperanto, direct objects take the -n ending.

• plumo = a pen
• plumon = a pen, as a direct object

Since alia describes plumon, it also gets -n:

• alia plumo = another pen
• alian plumon = another pen, as the thing being used

So:

• mi uzos alian plumon = I will use another pen

Because adjectives must agree with the nouns they describe.

Here:

• alia is an adjective meaning other / another
• plumon is a noun in the accusative

Since the noun has -n, the adjective must match it:

• alia plumo
• alian plumon

This agreement happens for number and case in Esperanto.

They are the same word, but in different grammatical forms.

• alia = base form
• alia-n = accusative form

The -n does not change the basic meaning another. It only shows grammatical function.

So:

• Mi havas alian plumon. = I have another pen.
• Alia plumo estas sur la tablo. = Another pen is on the table.

Because in ĉi tiu plumo ne funkcias, plumo is the subject, not the object.

The thing doing the action or being described stays without -n:

• ĉi tiu plumo = the subject
• ne funkcias = does not work

But in the second part, alian plumon is the object of uzos, so it takes -n.

Usually, uzos is the best choice here.

• mi uzos = I will use
• mi uzus = I would use

This sentence describes a normal, real possibility, so future tense fits:

• Se ĉi tiu plumo ne funkcias, mi uzos alian plumon.

If you said mi uzus, it would sound more hypothetical or dependent on some further condition. For a straightforward if this happens, I’ll do that sentence, uzos is standard.

Not completely. Esperanto word order is fairly flexible, because endings show the grammar.

The given sentence is the most natural and clear:

• Se ĉi tiu plumo ne funkcias, mi uzos alian plumon.

But some variation is possible, for example:

• Mi uzos alian plumon, se ĉi tiu plumo ne funkcias.

That means the same thing.

Still, the original order is very common because it presents the condition first and then the result.

In Esperanto, ne usually goes directly before the word or part it negates.

So:

• ne funkcias = does not work

That is the normal way to negate a verb phrase.

In this sentence, alia most naturally means another.

• alian plumon = another pen

It can also carry the broader sense other or different, depending on context. But here, since the speaker is replacing one pen with a second one, another pen is the most natural interpretation.