Mi petis interŝanĝon hieraŭ.

Because -is is the Esperanto past-tense ending.

• peti = to ask/request
• petis = asked/requested

So Mi petis... tells you the asking happened in the past.

The -n marks the direct object.

In this sentence, interŝanĝon is the thing being requested, so it takes the accusative ending:

• interŝanĝo = an exchange
• interŝanĝon = an exchange, as the direct object

Esperanto uses -n very regularly for direct objects.

Normally, no.

With petis, the noun is the direct object, so -n is expected:

• Mi petis interŝanĝon. = correct

Without -n, the sentence would sound ungrammatical in standard Esperanto.

It is built from smaller meaningful parts:

• inter- = between, among
• ŝanĝ- = change
• -o = noun ending

So interŝanĝo literally suggests something like a mutual change/exchange.

This is very typical of Esperanto word-building.

Because Esperanto has no indefinite article.

English says a/an, but Esperanto usually just uses the noun by itself:

• interŝanĝo = an exchange / exchange

Esperanto does have a definite article, la, for the. So if you meant a specific exchange already known from context, you could say la interŝanĝon.

This is a very common question.

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

So here petis interŝanĝon means requested an exchange, not asked a question about an exchange.

Examples:

• Mi petis helpon. = I asked for help.
• Mi demandis demandon. = I asked a question.

English uses ask for both ideas, but Esperanto usually separates them.

Esperanto word order is fairly flexible, so hieraŭ can move around.

All of these are possible:

• Mi petis interŝanĝon hieraŭ.
• Mi hieraŭ petis interŝanĝon.
• Hieraŭ mi petis interŝanĝon.

The meaning stays basically the same, though the emphasis may shift a little. Putting hieraŭ first often gives it extra focus.

Usually you should keep Mi.

Esperanto verbs show tense, but not person. That means petis could mean:

• I asked
• you asked
• he asked
• she asked
• we asked
• they asked

So the subject pronoun is normally needed unless the subject is already very clear from context.

Not by itself.

The ending -is only tells you the action was in the past. It does not automatically mark English-style aspect distinctions like was asking, asked, or had asked.

In many cases, Esperanto relies on context or extra words if you want to be more specific.

So Mi petis interŝanĝon hieraŭ simply places the requesting in the past.

Yes. That is basically the same idea.

peti often corresponds to English ask for or request. The exact English wording depends on style and context.

So this Esperanto sentence can fit either kind of translation:

• I asked for an exchange yesterday.
• I requested an exchange yesterday.

Not quite.

• Mi petis interŝanĝon = I requested an exchange.
• Mi petis pri interŝanĝo = I asked about an exchange / I made a request concerning an exchange, depending on context

Using the direct object interŝanĝon is the clearest way to say that the exchange itself is what you requested.

A rough guide is:

in-ter-SHAN-jon

Useful parts:

• ŝ sounds like English sh
• ĝ sounds like the g in gentle or judge
• j sounds like English y

So ŝanĝ sounds roughly like shanj.