Mia amikino pruntis al mi pluvombrelon hieraŭ, kaj mi redonos ĝin al ŝi morgaŭ matene.

Amikino is built from:

• amik- = friend
• -in- = female
• -o = noun ending

So amikino means female friend.
This -in- ending is very common in Esperanto:

• patro = father, patrino = mother
• kato = cat, katino = female cat

Because mia amikino is the subject of the first clause — the person doing the action.

In Esperanto, adjectives and possessives agree with the noun in number and case. Here:

• mia = my
• amikino = female friend

Both are singular and not accusative, so they stay as mia amikino.

If it were the direct object, you would see:

• mian amikinon

Esperanto has no indefinite article.
So English a friend, an umbrella, a book are usually just:

• amiko
• pluvombrelo
• libro

Esperanto does have la for the, but there is no separate word for a/an.

The -n marks the direct object.

Here, the thing being lent is the umbrella, so pluvombrelo becomes pluvombrelon.

Compare:

• La hundo vidas la katon. = The dog sees the cat.
• La kato vidas la hundon. = The cat sees the dog.

The -n helps show who is doing the action and what is receiving it.

Because mi is the recipient, not the direct object.

In Esperanto, a recipient is usually marked with al = to.

So:

• pruntis pluvombrelon al mi = lent an umbrella to me

If you said min, that would make me the direct object, which would not fit here.

English often says She lent me an umbrella, without to, but Esperanto normally shows this relationship explicitly with al.

This is a very common question.

Prunti can be understood from context, but the preposition makes the direction clear:

• prunti al iu = lend to someone
• prunti de iu = borrow from someone

So in this sentence:

• pruntis al mi = lent to me

Many speakers also use more explicit verbs:

• pruntedoni = lend
• pruntepreni = borrow

But in your sentence, al mi clearly shows that the friend was the lender.

Because Esperanto freely forms compound words.

• pluvo = rain
• ombrelo = umbrella / parasol

Together, pluvombrelo literally means something like rain-umbrella, i.e. umbrella.

This is very normal in Esperanto. The last main element usually determines the basic category of the word.

Redonos breaks down like this:

• re- = again / back
• don- = give
• -os = future tense

So redonos means will give back or will return.

That is why the sentence uses redonos instead of just donos:

• donos = will give
• redonos = will give back / return

Ĝin means it, and it refers back to pluvombrelon.

It has the -n because it is the direct object of redonos:

• mi redonos ĝin = I will return it

So:

• ĝi = it
• ĝin = it, as a direct object

Esperanto uses ĝi for things, and unlike many European languages, nouns themselves do not have arbitrary grammatical gender.

Because ŝi is the person receiving the returned umbrella, so Esperanto uses al:

• al ŝi = to her

If you said ŝin, that would make her the direct object.

So compare:

• Mi vidas ŝin. = I see her.
• Mi donas ĝin al ŝi. = I give it to her.

Because Esperanto often uses time words directly as adverbs.

• hieraŭ = yesterday
• morgaŭ = tomorrow

So you can simply say:

• Mi venis hieraŭ.
• Mi venos morgaŭ.

No extra word like on is needed.

Because matene is an adverb.

• mateno = morning
• matene = in the morning / morningly

The -e ending makes it adverbial. So:

• morgaŭ matene = tomorrow morning

This is a very common Esperanto pattern:

• tage = by day / during the day
• nokte = at night
• somere = in summer

Not completely. Esperanto word order is fairly flexible because endings like -n and prepositions like al show the roles of the words.

This sentence uses a very natural, straightforward order:

• subject
• verb
• recipient / object
• time

But you could also say, for example:

• Hieraŭ mia amikino pruntis al mi pluvombrelon, kaj morgaŭ matene mi redonos ĝin al ŝi.

That said, the original order is clear and idiomatic, so it is a good model for learners.