Se la pasvorto denove mankos, mi petos novan per retpoŝto.

Esperanto does not follow the English rule that an if clause normally uses present tense for future meaning.

In Esperanto, se just means if, and the tense shows the actual time of the action. Since this sentence talks about a possible future situation, mankos is future tense:

• Se la pasvorto denove mankos, mi petos novan...
  = If the password is missing again / will be missing again, I will request a new one.

So this is completely normal Esperanto. English and Esperanto simply handle future time differently here.

Manki means to be lacking, to be absent, or to be missing.

A useful thing to notice is that in Esperanto, the thing that is missing is often the subject:

• La pasvorto mankas = The password is missing.

If you want to mention the person affected, Esperanto often uses al:

• Al mi mankas la pasvorto = I am missing the password / The password is lacking to me.

In your sentence, the important idea is simply that the password is unavailable or absent again, so la pasvorto is the subject of mankos.

Because novan is the direct object of petos.

The full idea is really:

• mi petos novan pasvorton = I will request a new password

But Esperanto often leaves out a noun if it is obvious from context. So novan by itself means a new one, with pasvorton understood.

Since the hidden noun would be an object, the adjective also keeps the accusative -n:

• nova = new
• novan = new one / a new one, as a direct object

So mi petos novan means I will request a new one.

Usually, no. Peti and demandi are different.

• peti = to request, to ask for something
• demandi = to ask a question

Here the speaker wants to obtain a replacement password, so peti is the correct verb:

• mi petos novan = I will request a new one

If you used demandi, it would sound more like asking a question, not making a request.

This is a very common point for English speakers, because English uses ask for both ideas.

Per means by means of, via, or through. It shows the method or medium used to do something.

So:

• per retpoŝto = by email / via email

That is why the sentence means the request will be made using email as the method.

You will often see per with tools, methods, and channels:

• per telefono = by phone
• per trajno = by train
• per la interreto = through the internet

So mi petos novan per retpoŝto means I will request a new one by email.

La is the definite article, meaning the.

It is used because the sentence is talking about a specific password already known from the situation, not just any password in general. For example, it might be the password for a certain account that both speaker and listener already have in mind.

So:

• la pasvorto = the password
• pasvorto = a password / password in general

Using la makes the sentence sound like a concrete, specific real-world situation.

Here denove most naturally goes with mankos:

• Se la pasvorto denove mankos...
  = If the password is missing again...

In other words, the password has apparently gone missing before, and the same problem may happen again.

Word order in Esperanto is fairly flexible, but placement still helps show what feels most natural. In this sentence, denove sits next to mankos, so it is understood with that idea.

If you wanted to stress that the requesting happens again, you would probably place denove differently, for example:

• Se la pasvorto mankos, mi denove petos novan per retpoŝto.

That would mean If the password is missing, I will again request a new one by email.

Yes, that is possible, but it changes the feel a little.

• Se la pasvorto denove mankos, mi petos novan...
  This is a straightforward future possibility.

• Se la pasvorto denove mankus, mi petus novan...
  This sounds more hypothetical: If the password were missing again, I would request a new one...

So the original sentence with -os is the normal choice for a practical future plan. The -us version is more conditional or hypothetical.

Yes, novan is enough.

Esperanto often lets an adjective stand in place of a noun when the noun is obvious from context. Here everyone already knows the missing noun is pasvorton, so repeating it is unnecessary.

Both are correct:

• mi petos novan pasvorton
• mi petos novan

The shorter version is natural and efficient, and it works much like English a new one.