Este gasto mensual es pequeño.

In Spanish, adjectives like este / esta must agree in gender and number with the noun they modify.

• gasto is a masculine singular noun.
• Therefore, you must use the masculine singular demonstrative: este.

Forms of este:

• este gasto – this expense (masculine singular)
• esta factura – this bill (feminine singular)
• estos gastos – these expenses (masculine plural)
• estas facturas – these bills (feminine plural)

gasto covers both:

1. A specific expense or cost (countable):

   • Este gasto mensual es pequeño. – This monthly expense is small.
   • Tengo muchos gastos este mes. – I have many expenses this month.

2. Spending in general (uncountable):

   • El gasto en comida es alto. – Spending on food is high.

So you can think of it as both “expense” (a particular item) and “spending” (in general), depending on context.

Spanish usually places descriptive adjectives after the noun, especially when they are giving a neutral, factual description:

• gasto mensual – literally expense monthly → monthly expense
• coche nuevo – new car
• trabajo duro – hard work

Adjectives can sometimes go before the noun, but that often adds a nuance (emotional, subjective, stylistic). Here, mensual is just specifying the type of expense, so it naturally goes after gasto.

mensual is an adjective meaning “monthly” (something that happens every month or is calculated per month).

• gasto mensual – monthly expense (adjective modifying a noun)

mensualmente is an adverb meaning “monthly / every month”:

• Pago la factura mensualmente. – I pay the bill monthly.

In your sentence you’re describing the noun (gasto), so you need the adjective mensual, not the adverb mensualmente.

ser (es) is used for more inherent, characteristic, or classifying descriptions.
estar (está) is used for states or conditions, often temporary or resulting from change.

Describing the size/importance of an expense as pequeño here is a general characterization of that expense, not a temporary state, so Spanish uses ser:

• Este gasto mensual es pequeño. – It is (by nature / in general) a small expense.

You might use estar with adjectives for temporary conditions:

• El gasto este mes está siendo muy alto. – This month the spending is being very high. (more temporary, about this period)

Native speakers would normally not say un gasto bajo. For expenses, bajo sounds odd.

Typical choices:

• Este gasto mensual es pequeño. – This monthly expense is small.
• Este gasto mensual es muy bajo. – This is possible, but much less common; it sounds a bit like a literal translation from English.
• More natural alternatives:
  • Este gasto mensual es muy poco. – This monthly spending is very little.
  • No es un gasto muy grande. – It’s not a very big expense.
  • Es un gasto bastante reducido. – It’s quite a low/limited expense.

In practice, pequeño / grande (or expressions with poco / mucho) are the usual way to talk about the “size” of an expense.

They’re related in meaning (both suggest “not much”), but they work differently:

• pequeño is an adjective describing a thing (the noun):

  • un gasto pequeño – a small expense

• poco can be:

  • A determiner/adjective before a noun:
    • poco gasto – little spending / few expenses
  • Or an adverb with a verb:
    • gasto poco al mes. – I spend little per month.

Your sentence focuses on classifying the expense itself as “small”, so pequeño is ideal:

• Este gasto mensual es pequeño. – This particular expense is small.

If you focus on how much you spend, you might say:

• Gasto poco al mes. – I spend little per month.

Adjectives agree with the noun in gender and number:

• Singular:
  • el gasto mensual – the monthly expense
• Plural:
  • los gastos mensuales – the monthly expenses

Both gasto and mensual become plural when you’re talking about more than one expense:

• Este gasto mensual es pequeño. – This monthly expense is small.
• Estos gastos mensuales son pequeños. – These monthly expenses are small.

Yes, you can. The difference is nuance:

• Este gasto mensual es pequeño.

  • este = this specific expense (the one we’re pointing at or have just mentioned).
  • More specific and deictic (linked to the immediate context).

• El gasto mensual es pequeño.

  • el = the monthly expense, understood in general or as a concept in the context.
  • Could refer to “our monthly expense (as we usually have it)” or to a known, established expense.

So este singles out one specific expense in the current context; el sounds a bit more general or impersonal.

You place muy directly before the adjective:

• Este gasto mensual es muy pequeño. – This monthly expense is very small.

Word order here is fixed:

• Subject + verb + muy
  • adjective
• Este gasto mensual
  • es
    • muy
      • pequeño

Yes, grammatically you can:

• Este gasto es pequeño. – This expense is small.

You just lose the “per month / monthly” information. Whether that’s okay depends on context:

• If you’ve already been talking about monthly figures, people will understand from context.
• If not, and it’s important that it’s per month, you should keep mensual.

In standard European Spanish:

• gasto → /ˈɡas.to/

  • ga like “gah”
  • sto like “sto” in “stop” but with a pure Spanish o.

• mensual → /menˈsual/

  • men like “men”
  • sual like “swahl” (one syllable), stress on sual: men-SUAL.

Full sentence:
Este gasto mensual es pequeño. → /ˈes.te ˈɡas.to menˈsual es peˈke.ɲo/
Stress falls on É-ste, GAS-to, sual, que- in pequeño.