Puella dicit crus sibi dolere, quia heri in scalis cecidit.

Because after dicit Latin often uses an indirect statement:

• a subject in the accusative
• plus an infinitive

So Puella dicit crus sibi dolere means literally something like The girl says her leg to hurt, which in normal English becomes The girl says that her leg hurts.

Latin usually does this instead of using a that-clause.

Here crus is accusative singular, because it is the subject of the infinitive dolere in indirect statement.

This can be confusing because crus is a neuter noun, and in Latin neuter nouns often have the same form in the nominative and accusative singular.

So:

• nominative singular: crus
• accusative singular: crus

Even though the form looks unchanged, its job in the sentence is accusative.

Sibi is dative singular.

With dolere, Latin often puts the person who feels the pain in the dative. So the structure is literally:

• crus = the leg
• sibi = to her / for her
• dolere = to be painful / to hurt

So crus sibi dolere is very literally the leg to be painful to her.

English usually says her leg hurts, but Latin often expresses it with this dative idea.

Because sibi is a reflexive pronoun. It refers back to the subject, here puella.

So sibi means to herself / for herself, and in smoother English here it comes out as her.

Latin uses sibi to show that it is the girl's own leg that hurts. If you used ei, it would be non-reflexive and would more naturally refer to someone else in many contexts.

Latin often does not use a possessive adjective with body parts when the meaning is already clear.

Instead of saying her leg, Latin can say something more like the leg is painful to her. The ownership is understood from sibi.

So:

• English prefers her leg
• Latin is very comfortable with the leg to her

That is why crus sibi dolere is perfectly normal.

Quia introduces a reason clause: because.

So the sentence gives the girl's statement first, and then explains the reason:

• Puella dicit crus sibi dolere
• quia heri in scalis cecidit

That is, she says her leg hurts because she fell yesterday on the stairs.

Cecidit is perfect tense from cadere, meaning fell.

The perfect is used because this is a completed past action: the fall happened yesterday and is over.

So:

• cecīdīt = she fell
• cadebat would mean she was falling / used to fall, which would not fit as well here

The word heri also strongly points to a completed event in the past.

Because the subject is understood from the context.

In the quia clause, Latin does not need to repeat puella if it is already clear. So cecīdīt means she fell, and the natural subject is the previously mentioned puella.

Also, crus cannot be the subject of cecīdīt in any sensible meaning here. A leg does not fall down the stairs by itself in this context; the girl does.

In scalis means on the stairs or on the steps.

Scalae is commonly used in the plural to mean stairs / staircase. English also often treats stairs as a plural-looking word, so this idea is not too strange.

So:

• scala can mean a ladder or stair
• scalae often means stairs

Here scalis is ablative plural.

Because in with the ablative usually expresses location: in/on.

Here the idea is that the falling happened on the stairs, so Latin uses:

• in + ablative = location

If Latin wanted to emphasize motion into something, it would more typically use in + accusative. But here the stairs are treated as the place where the accident happened.

No. Latin word order is much more flexible than English word order, because the endings show the grammatical relationships.

So this sentence could be rearranged in various ways without changing the basic meaning. For example, Latin can move words around for emphasis.

The given order is natural:

• Puella dicit sets up the statement
• crus sibi dolere gives the content of what she says
• quia heri in scalis cecidit gives the reason

So the word order is meaningful and natural, but not rigidly fixed the way it would be in English.