Puella unum granum in manu tenet et rogat cur tam parvum sit.

Puella is nominative singular, and it is the subject of both tenet and rogat.

So the sentence is structured like this:

• puella = the girl
• tenet = holds
• rogat = asks

Latin often leaves the subject in the nominative case, just as English uses the plain form the girl rather than her.

Because granum is a neuter noun.

In Latin, adjectives and adjective-like words must agree with the noun they describe in:

• gender
• number
• case

Here:

• granum = neuter singular accusative
• unum = neuter singular accusative of unus

So unum granum means one grain.

A masculine form like unus would not match granum, and granus is not the correct noun here.

Granum is accusative singular because it is the direct object of tenet.

The girl is doing the action of holding, and the thing being held is the grain:

• puella = subject
• granum = direct object

That is why Latin uses the accusative here.

Because in can take either the ablative or the accusative, depending on the meaning.

• in + ablative = in/on a place, with no motion into it
• in + accusative = into a place, showing motion toward it

Here the grain is already in the hand, so Latin uses the ablative:

• in manu = in her hand

If the meaning were into the hand, then you would expect in manum.

Manus is a fourth-declension noun, and its forms are a little different from the more familiar first- and second-declension patterns.

Important forms:

• manus = nominative singular
• manus = genitive singular
• manui = dative singular
• manu = ablative singular
• manum = accusative singular

So in in manu, the noun is ablative singular, which is exactly the form needed after in for location.

Tam means so or so very.

So:

• tam parvum = so small

It is often used to strengthen an adjective or adverb.
Here the girl is asking why the grain is so small, not just small.

Because parvum describes granum, which is neuter singular.

Again, adjectives must agree with the noun they describe:

• parvus = masculine singular
• parva = feminine singular
• parvum = neuter singular

Since granum is neuter singular, parvum is the correct form.

Because cur tam parvum sit is an indirect question, and Latin normally uses the subjunctive in indirect questions.

Compare:

• direct question: Cur tam parvum est? = Why is it so small?
• indirect question: rogat cur tam parvum sit = She asks why it is so small

After verbs like rogat, quaerit, nescit, and similar verbs of asking or knowing, Latin usually puts the verb of the indirect question into the subjunctive.

So sit is the present subjunctive of esse.

It is a question reported inside a larger sentence rather than asked directly.

Direct question:

• Cur tam parvum est?
• Why is it so small?

Indirect question:

• Puella rogat cur tam parvum sit.
• The girl asks why it is so small.

The clause begins with cur and depends on rogat.
That dependence is what makes it indirect.

It can do both.

Rogare can mean:

• to ask someone
• to ask a question
• to ask for something

Here it introduces an indirect question:

• rogat cur... sit = she asks why... it is

So there does not need to be a separate person named in the sentence.

Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

English depends heavily on position:

• The girl holds one grain in her hand

Latin can move words around more freely because forms like puella, granum, and manu already show their roles.

This sentence is quite natural Latin:

• Puella unum granum in manu tenet et rogat cur tam parvum sit.

A few tendencies you can notice:

• the verb often comes later in the clause
• the indirect question comes after rogat
• the verb of the subordinate clause, sit, comes at the end

So the order is not random, but it is less rigid than English.

It can do both.

At a basic level, unum granum means one grain.
But in context, unum can also suggest a single grain or just one grain, especially if that matters to the scene.

Latin numerals often carry a little more force than a plain English one, depending on context.

Yes. Cur is a very common word meaning why.

So:

• cur? = why?
• cur tam parvum sit = why it is so small

It is one of the standard question words learners meet early, along with words like:

• quis = who?
• quid = what?
• ubi = where?
• quando = when?