En el prado cerca del arroyo, vimos una piedra grande y mucha hierba mojada.

Because de + el normally contracts to del in Spanish.

• de el arroyo → del arroyo
• This is a standard contraction.
• The only common exception is when el is part of a proper name, such as de El Escorial.

So cerca del arroyo means near the stream.

Vimos is the preterite form of ver (to see) for nosotros/nosotras (we).

So:

• ver = to see
• vimos = we saw

The preterite is used here because it describes a completed action in the past: we saw.

Because vimos presents the seeing as a completed event: we saw.

Compare:

• vimos una piedra... = we saw a stone...
• veíamos una piedra... = we were seeing / used to see a stone...

In this sentence, the speaker is describing one specific moment or event, so the preterite vimos is the natural choice.

In Spanish, adjectives usually come after the noun.

So:

• una piedra grande = a big stone

Putting grande before the noun is unusual here and would sound more literary or stylistically marked. For a basic physical description, Spanish normally uses noun + adjective.

Because hierba here is being treated as an uncountable noun, like grass in English.

So:

• mucha hierba = a lot of grass

If you said muchas hierbas, it would usually mean many herbs/plants as separate items, not grass as a mass covering the ground.

Because mojada agrees with hierba.

• hierba is feminine singular
• so the adjective must also be feminine singular

That gives:

• hierba mojada = wet grass

It is not mojadas because hierba is singular here, even though it refers to grass in a general mass sense.

It describes only hierba.

The structure is:

• una piedra grande = a big stone
• y mucha hierba mojada = and a lot of wet grass

So grande goes with piedra, and mojada goes with hierba.

If both the stone and the grass were wet, Spanish would normally make that clearer in a different way.

Spanish word order is flexible, and place expressions are often placed at the beginning to set the scene.

So starting with:

• En el prado cerca del arroyo

creates a natural background frame: In the meadow near the stream...

You could also say:

• Vimos una piedra grande y mucha hierba mojada en el prado cerca del arroyo

That is still grammatical, but it shifts the focus slightly.

Spanish often uses the definite article where English may or may not use the depending on style.

Here, el prado and el arroyo refer to a specific meadow and stream in the context of the story or scene.

So:

• en el prado = in the meadow
• cerca del arroyo = near the stream

This sounds natural in Spanish.

They are related, but not identical:

• prado = meadow, grassy field
• pradera = prairie/grassland/meadow, often broader or more expansive
• campo = countryside, field, rural area; much more general
• césped = lawn/grass turf, especially cut grass in a garden or park

In this sentence, prado fits well because it suggests a natural grassy meadow.

Arroyo usually means stream, brook, or a small watercourse.

It is generally smaller than:

• río = river

So arroyo gives a more specific image of a small stream rather than a large river.

Y means and and simply joins the two things that were seen:

• una piedra grande
• mucha hierba mojada

So:

• una piedra grande y mucha hierba mojada = a big stone and a lot of wet grass

A small extra point: y changes to e before words that begin with an i sound, but that does not apply here.

The h in Spanish is silent, so hierba is not pronounced with an English h sound.

A rough guide:

• hierba ≈ YER-ba

Also, in Spain, pronunciation can vary slightly by region, but the silent h is standard.

The comma separates the opening location phrase from the main clause.

So the sentence is structured like this:

• En el prado cerca del arroyo, = setting the scene
• vimos una piedra grande y mucha hierba mojada. = main action

This comma helps readability and is very natural when a longer introductory phrase comes first.