Learn Hardcore Esperanto: Hieraŭ mia fratino iris al kuracisto, ĉar ŝi estis malsana kaj havis doloron en la gorĝo. - Yesterday my sister went to the doctor because she was ill and had pain in her throat.

All of these are correct and mean the same thing. Putting Hieraŭ at the beginning simply emphasizes the time a bit more, similar to English: "Yesterday, my sister went to the doctor."

What kind of word is hieraŭ, and why doesn’t it have an ending like -o or -a?

Hieraŭ is an adverb (like “yesterday”, “today”, “quickly” in English).
Many basic adverbs in Esperanto don’t use the -e ending and are just “root adverbs,” for example:

hodiaŭ – today
morgaŭ – tomorrow
nun – now
ĉiujare = every year (this one uses the regular -e, but hieraŭ does not)

So hieraŭ is simply an adverb with its own fixed form; you don’t add -o, -a, or -e to it.

Why is it mia fratino and not la mia fratino?

In Esperanto, you normally do not use the definite article la together with a possessive adjective (mia, via, lia, ŝia, nia, via, ilia).
So you say:

mia fratino – my sister
via libro – your book

Using la mia fratino would sound strange and is almost never correct in normal sentences. (You can say la mia when you use it as a standalone noun, like “mine”: Tiu estas la mia. – “That one is mine.”)

Why is it iris al kuracisto instead of just iris kuraciston?

The verb iri (“to go”) normally needs a preposition to show the direction, usually al (“to, towards”).
So Esperanto says:

iri al la urbo – to go to the city
iri al amiko – to go to a friend
iri al kuracisto – to go to a doctor

If you said iris kuraciston, it would sound like kuraciston is a direct object (as if she “went a doctor”), which doesn’t make sense. The preposition al is needed to show direction clearly.

Why is there no la in al kuracisto? In English we say “to the doctor”.

Esperanto does not always use the definite article in the same places as English.
In al kuracisto, we are talking about a doctor in general, not a specific one previously mentioned, so no la is needed.

You could say al la kuracisto if you mean a specific doctor already known from the context (for example, her usual doctor mentioned earlier), but in a neutral sentence like this, al kuracisto is natural: “to (a) doctor” / “to see the doctor.”

Why is it ŝi estis malsana instead of using a simple verb like malsanis?

In Esperanto, the normal, idiomatic way to say “was ill / was sick” is:

ŝi estis malsana – she was ill

Here, estis is “was” and malsana is an adjective, “ill, sick.”

The verb malsani exists but usually means “to fall ill / to become sick”, not simply “to be in a state of illness.” You’re more likely to see it in sentences like:

Ŝi malsanis hieraŭ. – She fell ill yesterday.

So estis malsana describes the state; malsani describes the process of becoming ill.

Why does malsana end in -a and not -o or -j?

Malsana is an adjective. In Esperanto:

nouns end in -o (e.g. fratino, kuracisto, doloro)
adjectives end in -a (e.g. bela, granda, malsana)

Adjectives only take -j for plural (malsanaj) when they are describing a plural noun:

La infanoj estas malsanaj. – The children are ill.

Here, the subject ŝi is singular, so the adjective is singular: ŝi estis malsana.

Why is it havis doloron and not something like doloris or estis doloro?

There are several natural ways to say this idea in Esperanto:

ŝi havis doloron en la gorĝo – she had pain in her throat
ŝi sentis doloron en la gorĝo – she felt pain in her throat
ŝia gorĝo doloris – her throat hurt / was painful

The sentence uses havis doloron, literally “had a pain,” which is very clear and easy for learners: havi (to have) + doloro (pain).
Using doloris is also correct, but it changes the structure: the subject becomes the part of the body (gorĝo), not the person.

Why does doloron have the -n ending?

Doloron is the direct object of the verb havis (“had”).
In Esperanto, direct objects take the accusative -n:

Mi havas libron. – I have a book.
Ŝi havas hundon. – She has a dog.
Ŝi havis doloron. – She had pain.

So doloro (pain) → doloron (pain as object of the verb).

Why is it en la gorĝo, and not just en gorĝo or en ŝia gorĝo?

Using la with body parts is very common and natural in Esperanto, especially when it’s clear whose body we are talking about from the context. So:

Li rompis la brakon. – He broke (his) arm.
Ŝi havis doloron en la gorĝo. – She had pain in (her) throat.

You can say en ŝia gorĝo, but it often sounds unnecessary, because it’s already clear that the throat belongs to ŝi.

Saying en gorĝo without la is grammatically possible but would sound incomplete or strange in this context; en la gorĝo is the normal idiomatic choice.

Why doesn’t gorĝo also take -n, like doloron?

Gorĝo is part of the prepositional phrase en la gorĝo (“in the throat”).
In Esperanto, nouns after most prepositions do not take the accusative ending -n, unless you are showing movement into something (which is not the case here):

en la gorĝo – in the throat (location, no -n)
en la domon – into the house (movement, here -n is possible and usual)

So here we are talking about where the pain is (location), not movement, so it’s simply en la gorĝo, no -n.

Why is there a comma before ĉar, and is it always required?

The comma before ĉar (“because”) is stylistic but very common.
Esperanto usually uses a comma before conjunctions that introduce a subordinate clause, such as:

ĉar – because
ke – that
se – if

So writers often write:

…iris al kuracisto, ĉar ŝi estis malsana…

You will also see it without the comma, and it’s not “wrong,” but the comma helps show that a new clause (the reason) is beginning.

Why is it kaj havis doloron and not kaj ŝi havis doloron?

In Esperanto, when two verbs share the same subject, you don’t have to repeat the subject the second time if it’s clear:

Ŝi estis malsana kaj havis doloron.
(literally: “She was ill and had pain.”)

The subject ŝi applies to both estis and havis, so repeating ŝi is optional.
You could say ĉar ŝi estis malsana kaj ŝi havis doloron en la gorĝo, and it would still be correct, but the shorter version without repeating ŝi sounds more natural.