Esta marcha es muy corta.

Because marcha is a feminine singular noun.

In Spanish, words like this must agree with the noun they go with:

• este = masculine singular
• esta = feminine singular
• estos = masculine plural
• estas = feminine plural

So:

• esta marcha
• but este libro

Even though this does not change in English, Spanish demonstratives do.

For the same reason: corta has to agree with marcha, which is feminine singular.

Spanish adjectives usually change to match the noun:

• marcha corta = short march/walk
• viaje corto = short trip

Here the pattern is:

• marcha = feminine singular
• therefore corta = feminine singular

If the noun were plural, it would become cortas:

• Estas marchas son muy cortas.

Because Spanish normally uses ser for describing something’s general characteristic, such as its length, size, or nature.

So:

• La película es corta.
• El camino es largo.
• Esta marcha es muy corta.

Using estar here would sound unnatural in standard Spanish, because short is being treated as a characteristic of the march itself, not as a temporary condition.

Because muy is the word used before an adjective or adverb to mean very.

Here, corta is an adjective, so you say:

• muy corta = very short

Compare:

• muy corto = very short
• mucho tiempo = a lot of time
• muchas marchas = many marches/walks

A useful shortcut:

• muy = very
• mucho / mucha / muchos / muchas = much / a lot of / many

Also, muy does not change for gender or number.

Marcha can mean different things depending on context. Common meanings include:

• a march
• a walk or organised walk
• a hike/route in some contexts
• in other situations, even party atmosphere or nightlife

In this sentence, the context tells you which meaning is intended. Grammatically, the important thing is that marcha is a feminine noun, which is why you get esta and corta.

Here it is a demonstrative adjective, because it comes directly before a noun:

• esta marcha

It is describing which march.

If it stood alone, replacing the noun, it would be a pronoun:

• Esta es muy corta. = This one is very short.

So in your sentence, esta is not standing alone; it modifies marcha.

In modern standard Spanish, normally no.

The recommended spelling today is:

• esta marcha

Older materials may show ésta when it is used as a pronoun, but current standard spelling usually drops that accent except in rare cases of ambiguity.

So for learners, the safest rule is:

• write esta, not ésta

And in your sentence, since it is an adjective before a noun, it would be esta anyway.

A simple English-friendly guide is:

EH-sta MAR-cha es mwee KOR-ta

A few points:

• e in es is like eh
• ch in marcha is like ch in church
• c in corta is a hard k sound
• r in corta is tapped lightly, not like a strong English r
• muy is usually pronounced roughly mwee or moo-ee, depending on speaker and speed

A more Spanish-like rhythm would be:

EH-sta MAR-cha ez mwee KOR-ta

Because esta already does the job of pointing out the noun: this march/walk.

In Spanish, you normally do not use an article together with a demonstrative in this kind of structure.

So you say:

• esta marcha
• not la esta marcha

Compare:

• la marcha = the march
• esta marcha = this march

Yes. That would simply mean This march is short.

Adding muy makes it stronger:

• Esta marcha es corta. = This march is short.
• Esta marcha es muy corta. = This march is very short.

The basic sentence structure stays the same.

Yes, but the meaning changes slightly.

• Esta marcha es muy corta = this march is very short
• La marcha es muy corta = the march is very short

So the difference is not grammar of the rest of the sentence, but which determiner you use:

• esta points to something near or already identified as this one
• la means the

The structure is:

• Esta marcha = subject
• es = verb
• muy corta = description

So it follows a very common Spanish pattern:

subject + ser + adjective

Examples:

• El libro es interesante.
• La película es larga.
• Esta marcha es muy corta.

This is very similar to English word order in this kind of sentence.