Correr en la arena me cansa mucho.

Because correr is being used as an infinitive meaning running.

In Spanish, an infinitive can act like a noun:

• Correr es divertido. = Running is fun.
• Nadar me relaja. = Swimming relaxes me.

So in Correr en la arena me cansa mucho, the whole idea running on the sand is the subject of the sentence.

Because cansa agrees with the subject, which is correr en la arena.

Here is the structure:

• Correr en la arena = the thing that causes the effect
• me = the person affected
• cansa = tires

So literally, it is something like:

• Running on the sand tires me a lot

If you said me canso, that would mean I get tired:

• Me canso mucho corriendo en la arena. = I get very tired running on the sand.

Both are possible in Spanish, but they mean slightly different things.

Me is an indirect/object pronoun showing who is affected: me.

Compare:

• Me cansa = It tires me
• Te cansa = It tires you
• Lo cansa / La cansa = It tires him/her
• Nos cansa = It tires us

So me cansa mucho means it tires me a lot.

It is singular because the subject is treated as one activity: correr en la arena.

An infinitive phrase usually takes a singular verb:

• Fumar es malo.
• Leer me ayuda.
• Correr en la arena me cansa.

Even though the idea may feel broad in English, Spanish sees it as one single action/activity.

En is the normal preposition here for on or in, depending on the context.

With surfaces or environments, Spanish often uses en where English uses on:

• en la cama = on the bed
• en la playa = on the beach
• en la arena = on the sand

So correr en la arena is the natural way to say to run on the sand.

Spanish often uses the definite article more than English does.

Here, la arena means the sand in a general, natural-context sense, not necessarily one specific pile of sand. It sounds more natural than just arena in this sentence.

So:

• correr en la arena = natural
• correr en arena = much less natural here

This is similar to:

• caminar por la playa
• tumbarse en la hierba

Mucho modifies the verb phrase me cansa and means a lot or very much.

So:

• me cansa mucho = it tires me a lot

Putting mucho after the verb is very common in Spanish:

• Me gusta mucho
• Trabaja mucho
• Me preocupa mucho

Its position at the end is completely normal.

Yes, and it changes the nuance slightly.

• en la arena = on the sand, focusing on the surface
• por la arena = across/through the sand, focusing more on movement through that area

Both can be correct, but en la arena is a very natural choice when talking about the effort of running on sand as a surface.

Yes, but it means something slightly different.

• en la arena = specifically on the sand
• en la playa = on the beach

A beach includes more than just sand, so en la arena is more precise if the difficulty comes from the sand itself.

Normally, it expresses a general fact or usual experience:

• Correr en la arena me cansa mucho. = Running on sand tires me a lot.

It sounds like a statement about what is generally true for the speaker.

If you wanted to emphasize right now, Spanish would often use a different structure, for example:

• Correr en la arena me está cansando mucho. = Running on the sand is tiring me a lot.

Yes, but the original order is very natural.

Original:

• Correr en la arena me cansa mucho.

Possible variation:

• Me cansa mucho correr en la arena.

Both are correct. The difference is mainly in emphasis:

• Starting with Correr en la arena highlights the activity itself.
• Starting with Me cansa mucho highlights the effect on the speaker.

Very often, yes. Cansar commonly means to tire or to make someone tired.

Pattern:

• algo/alguien + cansar + a alguien

Examples:

• Este trabajo me cansa. = This job tires me.
• Los niños nos cansan. = The children tire us.
• Caminar tanto te cansa. = Walking so much tires you.

There is also the reflexive form:

• cansarse = to get tired
• Me canso rápido. = I get tired quickly.

So:

• me cansa = it tires me
• me canso = I get tired