Książka spadła z półki.

Because książka is the subject of the sentence, so it is in the nominative singular. This is also the dictionary form of the noun.

• książka = nominative singular
• it is a feminine noun
• in this sentence, it is the thing that did the falling

If the book were an object rather than the subject, the form might be different.

In the Polish past tense, the verb agrees with the subject in gender and number.

Since książka is:

• singular
• feminine

the verb must be spadła.

Compare:

• spadł = he / a masculine thing fell
• spadła = she / a feminine thing fell
• spadło = it fell, for a neuter noun

So Książka spadła is grammatically matched.

The basic dictionary form is spaść, which means to fall in the sense of a completed fall.

spadła is the past tense, feminine singular form of spaść.

A very rough breakdown is:

• spaść = infinitive
• spadł / spadła / spadło = past tense forms

This stem change is normal and must simply be learned:

• spaść → spadł(a/o)

Because the preposition z meaning from / off requires the genitive case.

So:

• półka = shelf, nominative
• półki = shelf, genitive singular

After z in this meaning, you need the genitive:

• z półki = from the shelf / off the shelf

This is a very common pattern in Polish:

• z domu = from the house
• z pracy = from work
• z półki = from the shelf

It can correspond to both in English, depending on context.

In Książka spadła z półki, English would often say:

• from the shelf
• or more naturally, off the shelf

Polish uses z for this idea of movement away from a surface or place.

So the Polish is natural even if English chooses either from or off.

Because Polish does not have articles like English a and the.

So Książka spadła z półki can mean:

• The book fell off the shelf
• A book fell off the shelf

Which one is meant depends on context.

This is completely normal in Polish.

No. Polish word order is fairly flexible.

Książka spadła z półki is a neutral, natural order.

But you could also say:

• Z półki spadła książka.

That might be used if you want to emphasize where it fell from, or if the context already leads into that information.

Even though word order can change, the case endings still show the grammatical roles.

This is a question of aspect.

• spadła is perfective: it describes a single completed event
• spadała is imperfective: it would suggest was falling or kept falling

In this sentence, the fall is presented as a finished event, so spadła is the normal choice.

Compare:

• Książka spadła z półki. = the book fell off the shelf
• Książka spadała z półki. = the book was falling off the shelf

The second one sounds less natural by itself unless you are describing the action in progress.

A rough English-friendly guide is:

• książka ≈ kshonsh-ka
• spadła ≈ spad-wa
• z półki ≈ s pułki

A few useful details:

• ą is a nasal vowel, somewhat like on said through the nose
• ł is pronounced like English w
• ó is pronounced like u
• z before the voiceless p in półki is often pronounced like s

Also, Polish stress is usually on the second-to-last syllable:

• KSIĄŻ-ka
• SPAD-ła
• PÓŁ-ki

They are normal letters of the Polish alphabet, not optional accents.

• ą = a nasal vowel
• ł = pronounced like English w
• ó = pronounced like u

So:

• książka contains ą
• spadła contains ł
• półki contains ó and ł

You need to learn them as part of the spelling system, because they are distinct Polish letters.

Sometimes, but not always.

• spaść often fits well when something falls from somewhere
• upaść often means to fall down

In this sentence, because the book fell from/off the shelf, spadła is the most natural choice.

So:

• Książka spadła z półki. = very natural
• Książka upadła z półki. = possible in some contexts, but less typical here

For a learner, it is best to remember this sentence with spadła.