El gato se rasca mucho.

Se makes the verb reflexive here.

• rascar = to scratch something or someone
  
  • El gato rasca la puerta. = The cat scratches the door.
• rascarse = to scratch oneself
  
  • El gato se rasca. = The cat scratches itself.

So in El gato se rasca mucho, the cat is doing the action to itself.

Rasca is the third-person singular present tense form of rascarse (or rascar).

It matches el gato, which is grammatically he/it in Spanish.

Present tense of rascarse:

• yo me rasco
• tú te rascas
• él/ella se rasca
• nosotros/as nos rascamos
• ustedes se rascan
• ellos/ellas se rascan

So:

• El gato se rasca = The cat scratches itself

Spanish often uses the definite article more than English does.

El gato can mean:

• the cat (a specific cat), or sometimes
• a general subject in context, depending on the situation

In this sentence, El gato sounds natural as the subject. Saying just Gato se rasca mucho would not be normal standard Spanish.

Because mucho here modifies a verb, not an adjective or adverb.

• mucho = a lot / a great deal
• muy = very

So:

• El gato se rasca mucho = The cat scratches itself a lot
• not very scratches itself

Compare:

• El gato es muy inquieto. = The cat is very restless.
• El gato se mueve mucho. = The cat moves a lot.

Because in this sentence mucho is an adverb, not an adjective.

When mucho modifies a verb, it stays mucho:

• duerme mucho
• come mucho
• se rasca mucho

But when it modifies a noun, it agrees in gender and number:

• mucho pelo = a lot of hair
• mucha agua = a lot of water
• muchos gatos = many cats
• muchas pulgas = many fleas

Not exactly.

• El gato se rasca mucho = The cat scratches itself a lot
• El gato rasca mucho = The cat scratches a lot

Without se, it sounds like the cat scratches in general, or scratches things/someone, not necessarily itself.

So if you want the clear idea of scratching itself, se is important.

With a conjugated verb, the reflexive pronoun usually goes before the verb:

• El gato se rasca mucho.
• El perro se lame.

With an infinitive or gerund, it can attach to the end:

• El gato quiere rascarse.
• El gato está rascándose.

With an affirmative command, it also attaches:

• Ráscate. = Scratch yourself.

Not literally. It literally means:

• The cat scratches itself a lot

That often suggests the cat is itchy, has fleas, or is uncomfortable, but the sentence itself only states the scratching.

If you want to say the cat is itchy, Spanish might use other expressions depending on region, such as:

• Al gato le pica mucho la piel.
• El gato tiene comezón. (common in some regions)
• Al gato le da mucha picazón.

Usually, yes, when someone scratches themselves.

Examples:

• Me rasco la cabeza. = I scratch my head.
• Se rasca el brazo. = He/She scratches his/her arm.

But rascar without se is used when the action is on something else:

• El gato rasca el sofá. = The cat scratches the sofa.

So the reflexive form is very common for body-related actions done to oneself.

Spanish usually does not need extra words like itself if the reflexive pronoun already makes the meaning clear.

• El gato se rasca already means The cat scratches itself

You could add emphasis in some contexts, but it is usually unnecessary.

Similarly, Spanish often uses:

• Me lavo las manos. = literally I wash myself the hands, meaning I wash my hands
• Se rasca la oreja. = It scratches its ear

So Spanish often prefers the reflexive structure instead of adding a separate word like itself.

A simple pronunciation guide is:

• rasca ≈ RAHS-kah
• gato ≈ GAH-toh
• se ≈ seh
• mucho ≈ MOO-choh

A rough full pronunciation:

• el GAH-toh seh RAHS-kah MOO-choh

Notes:

• The r in rasca is a soft tapped Spanish r
• The c in rasca sounds like k because it comes before a
• The ch in mucho sounds like English ch in church

Yes. That is also correct, but it means something slightly different.

• El gato se rasca mucho = The cat scratches itself a lot
  This can describe a general habit or what is happening.
• El gato está rascándose mucho = The cat is scratching itself a lot
  This emphasizes that it is happening right now.

In everyday Spanish, the simple present is often very common, even when English might prefer is scratching.