La ceniza cayó al suelo.

Ceniza can be used as a mass noun in Spanish, just like ash in English. So la ceniza means ash in a general, uncountable sense.

• La ceniza cayó al suelo = The ash fell to the ground
• Las cenizas usually means ashes, often referring to separate remains, such as after a fire or after cremation.

So the singular here is completely natural.

Cayó is the third-person singular preterite of caer (to fall).

That means it matches subjects like:

• él cayó = he fell
• ella cayó = she fell
• la ceniza cayó = the ash fell

The preterite is used here because it describes a completed event: the ash fell.

The accent mark shows both pronunciation and helps distinguish the correct written form.

• cayó = he/she/it fell
• caigo = I fall
• cae = he/she/it falls
• cayo without an accent is a different word in other contexts

In cayó, the stress falls on the last syllable: ca-YÓ.

Al is the contraction of a + el.

• a = to
• el suelo = the ground / the floor
• al suelo = to the ground / onto the floor

With verbs of movement like caer, Spanish often uses a to show the destination:

• cayó al suelo = fell to the ground

You generally would not say just cayó suelo.

In Spanish, a + el normally contracts to al.

So:

• a el suelo → al suelo

This contraction is required in normal Spanish, except in a few special cases, mainly when el is part of a proper name.

It can mean either, depending on context.

• indoors: usually floor
• outdoors: usually ground

So al suelo is a very common expression meaning to the floor / to the ground. Spanish uses the same noun in many contexts where English chooses different words.

Spanish uses articles more often than English.

In this sentence:

• la ceniza refers to the ash as the subject
• el suelo appears inside al suelo, literally to the ground

Even when English might sound more natural without an article in some contexts, Spanish often keeps it. This is one of those patterns learners need to get used to.

Usually, La ceniza cayó al suelo is the more natural choice.

Caer already means to fall, so se is not normally needed here.
Caerse does exist and is common, but it is used more often with things like people or objects falling, and it can sometimes suggest a more accidental or noticeable fall:

• El vaso se cayó al suelo = The glass fell onto the floor
• Me caí = I fell

With ceniza, plain cayó sounds simpler and more natural.

Yes, Cayó la ceniza al suelo is grammatically possible, but La ceniza cayó al suelo is the most neutral word order.

Spanish is more flexible with word order than English, but changing the order can slightly change the focus or style.

• La ceniza cayó al suelo = neutral statement
• Cayó la ceniza al suelo = more literary, dramatic, or focused on the action

For everyday learning, the original order is the safest pattern.

In standard Spanish from Spain, ceniza is pronounced roughly theh-NEE-tha.

Breakdown:

• ce sounds like theh in much of Spain
• ni sounds like nee
• za sounds like tha in much of Spain

So: ce-NI-za with stress on ni.

In many parts of Latin America, the z and c before e/i are pronounced like s, so it sounds more like seh-NEE-sa.