El mantenimiento cuesta mucho.

Because mantenimiento is a masculine noun in Spanish, so it takes el.

A useful clue is the ending: many nouns ending in -miento are masculine, for example:

• el movimiento
• el nacimiento
• el conocimiento

So el mantenimiento is the normal form.

Cuesta is the third person singular of the verb costar in the present tense.

Here is the relevant part of the conjugation:

• yo cuesto
• tú cuestas
• él / ella / usted cuesta

Since el mantenimiento is singular, the verb is singular too:

• El mantenimiento cuesta mucho.

If the subject were plural, you would use cuestan:

• Los mantenimientos cuestan mucho.
• Las reparaciones cuestan mucho.

Because Spanish uses the verb costar to talk about something costing money or being costly.

So Spanish says:

• El mantenimiento cuesta mucho.

not:

• El mantenimiento es mucho.
• El mantenimiento está mucho.

In English, we might say Maintenance is expensive or Maintenance costs a lot, but in Spanish the natural structure here is with costar.

Because mucho and muy do different jobs.

• mucho = a lot
• muy = very

With costar, Spanish normally says:

• cuesta mucho = costs a lot

If you want to use muy, you need an adjective:

• Es muy caro = It is very expensive

So these are both natural, but they are built differently:

• El mantenimiento cuesta mucho.
• El mantenimiento es muy caro.

Literally, mucho means a lot. In this sentence, it modifies cuesta, so the idea is:

• it costs a lot

Depending on context, the natural English translation might be:

• Maintenance costs a lot
• Maintenance is expensive

So mucho does not literally mean expensive, but the overall sentence can imply that idea.

Normally, no. In Spanish, abstract or general nouns often still take the definite article when talking about something in a general sense.

So:

• El mantenimiento cuesta mucho. = natural
• Mantenimiento cuesta mucho. = generally sounds incomplete or unnatural

Spanish uses articles more often than English does, so this is something English speakers often need to get used to.

The normal, neutral word order is:

• El mantenimiento cuesta mucho.

That is the most straightforward version.

You can also say:

• Cuesta mucho el mantenimiento.

But that sounds more marked or contextual, as if you are emphasizing the maintenance or responding to something already being discussed.

So for a learner, the safest default is:

• El mantenimiento cuesta mucho.

It can be used in many contexts. Mantenimiento is a broad word meaning maintenance, upkeep, or servicing, depending on the situation.

For example:

• el mantenimiento del coche = car maintenance / servicing
• el mantenimiento del edificio = building maintenance
• el mantenimiento de la piscina = pool maintenance
• el mantenimiento de la página web = website maintenance

So it is a very flexible noun.

A helpful approximation is:

• mahn-teh-nee-mee-EN-toh

The stress falls on -mien-:

• mante-ni-MIEN-to

A few pronunciation notes:

• a, e, i, o are pure vowels in Spanish, not diphthong-heavy like in English.
• t is usually crisper and lighter than English t.
• The r in this word is a single r, so it is a light tap, not a strong rolled rr.

Usually no, not with the same meaning.

• costar is the normal verb for to cost
• valer usually means to be worth

So:

• El mantenimiento cuesta mucho. = The maintenance costs a lot.
• El mantenimiento vale mucho. = The maintenance is worth a lot.

That second sentence would mean something different, and in many contexts it would sound odd. If you want to talk about price, use costar.

Then the verb changes to agree with the plural subject.

Compare:

• El mantenimiento cuesta mucho.
• Los repuestos cuestan mucho.
• Las reparaciones cuestan mucho.

So the rule is:

• singular subject → cuesta
• plural subject → cuestan

This is just normal subject-verb agreement in Spanish.