Cerca de la cascada crece tanta hierba que casi no se ve el sendero.

Spanish is often more flexible than English with word order. Starting with Cerca de la cascada puts the location first for emphasis, a bit like saying:

• Near the waterfall, so much grass grows that...

A more neutral order would also be possible:

• Tanta hierba crece cerca de la cascada que casi no se ve el sendero.

Both are grammatical, but the original sounds very natural and descriptive.

Cerca de is a fixed expression meaning near or close to.

Examples:

• cerca de la playa = near the beach
• cerca del pueblo = near the village

You need de because cerca on its own does not directly take a noun. So Spanish says:

• cerca de + noun

not just cerca la cascada.

In Spanish, articles like el, la, los, las are used more often than in English. When referring to a specific place or thing, Spanish usually includes the article.

So:

• la cascada = the waterfall
• el sendero = the path

Even when English might sometimes sound fine without an article in certain contexts, Spanish usually keeps it.

Because hierba is grammatically singular.

Even though grass is made up of many blades, Spanish treats hierba as a mass noun, like grass in English. So the verb agrees with hierba, not with the idea of quantity.

• La hierba crece = The grass grows
• Mucha hierba crece aquí = A lot of grass grows here

That is why it is crece, not crecen.

Tanta ... que means so much ... that.

It is a very common structure in Spanish for showing consequence:

• Hay tanta niebla que no veo nada.
  = There is so much fog that I can’t see anything.

• Tiene tantos libros que no caben en la estantería.
  = He has so many books that they don’t fit on the shelf.

In your sentence:

• tanta hierba que casi no se ve el sendero
  = so much grass that you can hardly see the path

Both are possible, but they mean slightly different things.

• mucha hierba = a lot of grass
• tanta hierba = so much grass

Tanta is used because the sentence continues with a consequence:

• so much grass that...

That is why tanta ... que fits perfectly here.

Also, tanta agrees with hierba, which is feminine singular:

• tanto for masculine singular
• tanta for feminine singular
• tantos / tantas for plural nouns

Se ve is a very common Spanish way to express something in an impersonal or passive-like way.

Here, casi no se ve el sendero means something like:

• the path can hardly be seen
• you can hardly see the path

Spanish often uses se when the subject is not important or when speaking generally.

Compare:

• No se ve la montaña. = The mountain can’t be seen / You can’t see the mountain.
• Se venden libros. = Books are sold.

So se ve keeps the sentence general and natural.

In this kind of se construction, el sendero functions like the grammatical subject of the verb.

That is why the verb is singular:

• se ve el sendero
• se ven las montañas

Compare:

• Se ve el sendero. = The path can be seen.
• Se ven los senderos. = The paths can be seen.

So the verb agrees with el sendero, not with an omitted person.

They are related, but not identical.

• sendero = path, trail, often narrow and natural
• camino = road, way, path; more general
• ruta = route, itinerary, route you follow

In this sentence, sendero is a good choice because it suggests a natural footpath near a waterfall.

Because casi normally goes before the word or idea it modifies.

• casi no se ve = it is almost not visible / you can hardly see it

This is the natural Spanish order.

No casi se ve sounds wrong in standard Spanish.

Very often, casi no works together as a unit meaning:

• hardly
• almost not

Not usually in this context.

• hierba = grass, wild grass, vegetation
• césped = lawn grass, especially cut and maintained grass
• pasto = pasture, grazing land, or animal feed; in some regions it can also mean grass, but it would sound less natural here in Spain

Near a waterfall, where grass grows naturally and thickly, hierba is the best fit.

Yes, crece is a very good choice because it gives a vivid image: the grass is actively growing there.

• Cerca de la cascada crece tanta hierba...
  = Near the waterfall, so much grass grows...

You could say:

• Hay tanta hierba cerca de la cascada que...
  = There is so much grass near the waterfall that...

That version is also correct, but crece sounds more descriptive and literary.

More or less, but a fully literal translation sounds a bit unusual in English.

Very literal:

• Near the waterfall grows so much grass that the path is almost not seen.

More natural English:

• So much grass grows near the waterfall that you can hardly see the path.
• Near the waterfall, so much grass grows that the path is almost invisible.

So the Spanish structure is natural, even if English usually rearranges it a bit.

It sounds descriptive and quite natural, with a slightly literary feel because of the word order and the use of crece.

It is not overly formal, but it is more elegant than a plain everyday version like:

• Hay mucha hierba cerca de la cascada y casi no se ve el sendero.

The original flows well and paints a picture, which is why it feels a bit richer stylistically.