Learn Hardcore Spanish (Latin America): Muchos adultos sienten estrés porque tienen deudas y piensan en la próxima crisis económica. - Many adults feel stressed because they have debts and think about the next economic crisis.

sentir estrés = to feel stress (focus on the emotion/feeling itself, as a thing they experience)
estar estresados = to be stressed (focus on the state/condition they are in)

In this sentence, sienten estrés sounds a bit more neutral and general: they experience stress.
Están estresados would also be correct, and often sounds a bit more like they are (currently) stressed out.

In Latin American Spanish, you’ll hear:

Siento estrés por el trabajo.
Estoy estresado por el trabajo.

They’re very close in meaning; the choice is mostly style and nuance, not grammar.

Why is there no article in sienten estrés? Why not sienten el estrés?

With many abstract, “mass” nouns (like stress, fear, hunger), Spanish often uses them without an article after certain verbs, especially sentir, tener, dar, etc.

So we commonly say:

sentir estrés / miedo / vergüenza
tener hambre / sed / miedo
dar miedo / vergüenza / pena

Sienten el estrés is not wrong, but it usually refers to some specific, identifiable stress (for example, the pressure of a particular situation):

Sienten el estrés del examen final.
They feel the stress of the final exam.

In your sentence, it’s “stress” in general, so sienten estrés (no article) is the most natural choice.

Why is it porque and not por qué?

porque (one word, no accent) = because (introduces a reason)
- Sienten estrés porque tienen deudas.
  They feel stress because they have debts.
por qué (two words, with accent) = why (used in direct and indirect questions)
- ¿Por qué sienten estrés?
  Why do they feel stress?
- No entiendo por qué sienten estrés.
  I don’t understand why they feel stress.

In your sentence we’re explaining the reason, not asking a question, so it must be porque.

Why is it tienen deudas in the plural, instead of tienen una deuda or tienen deuda?

All three forms exist, but they express slightly different ideas:

tienen deudas (plural, no article)
= they have debts / they are in debt in a general, non‑specific way. This is the most common way to say people owe money to various places.
tienen una deuda
= they have a debt (one specific debt, like a single loan or a single unpaid bill).
tienen deuda
This is less common and sounds a bit more technical or formal, like saying they have debt as a general financial condition (you’re more likely to hear it in news, economics, etc.).

In everyday speech, for people who owe money to different banks, credit cards, etc., tener deudas (plural) is the usual phrase.

Why do we say piensan en and not piensan sobre or piensan de?

Spanish distinguishes these quite clearly:

pensar en + algo/alguien = to think about something/someone
(the mental action of directing your thoughts toward it)
- Piensan en la próxima crisis económica.
  They think about the next economic crisis.
pensar sobre + tema = to think about (a topic), usually in the sense of considering or analyzing it as a subject of discussion or study
- Estoy pensando sobre el problema del cambio climático.
pensar de + algo = to think of in the sense of having an opinion about
- ¿Qué piensas de la película?
  What do you think of the movie?

In your sentence, they’re not being asked for an opinion; it’s just that their thoughts are focused on the crisis. So pensar en is the right choice.

Why do we say piensan en la próxima crisis económica and not piensan en la crisis económica próxima?

Both are grammatically possible, but the normal, most natural word order in Spanish is:

article + adjective like “próximo” + noun + descriptive adjective

So:

la próxima crisis económica is the default order.
la crisis económica próxima is more marked and unusual; it can sound more formal, technical, or written, and often implies “the economic crisis that is next (in a sequence)” rather than just “the next (upcoming) economic crisis”.

In everyday, natural speech for “the next economic crisis,” Spanish strongly prefers la próxima crisis económica.

Why is crisis feminine, and does it change in the plural?

Crisis is one of those Spanish nouns that:

is feminine → we say la crisis, una crisis
has the same form in singular and plural → la crisis / las crisis

So:

la crisis económica – the economic crisis
las crisis económicas – the economic crises

You can’t see the plural visually in the noun, only in the article and adjectives:

la próxima crisis económica
las próximas crisis económicas

Why is it Muchos adultos and not Muchos de los adultos?

Both are correct but mean different things:

Muchos adultos = many adults (in general), an indefinite, broad group.
That’s the meaning in your sentence: many adults in general feel stress.
Muchos de los adultos = many of the adults (from a specific group already known from context)
Example:
- En esta empresa, muchos de los adultos sienten estrés.
  In this company, many of the adults feel stress.

Since your sentence talks about adults in society in general, Muchos adultos is the natural choice.

Could we say Muchos adultos están estresados instead of sienten estrés? Would it change the meaning?

Yes, you can say that, and it’s very common:

Muchos adultos están estresados porque tienen deudas…

The difference is subtle:

sienten estrés → talks about the feeling of stress as something they experience.
están estresados → describes their state/condition (they are stressed).

In practice, in this type of general statement, both are acceptable.
If you want it to sound a bit more like “they’re under a lot of pressure right now,” están estresados is more direct.
If you want a slightly more neutral or descriptive tone, sienten estrés works well.

Why is there no comma before porque?

In Spanish, porque introducing a reason usually does not take a comma when it’s closely tied to the main clause:

Muchos adultos sienten estrés porque tienen deudas… ✅

You might add a comma if the explanation is more like an afterthought or if you want to emphasize it:

Muchos adultos sienten estrés, porque tienen deudas.
(Possible, but sounds more like you’re adding the reason as an extra comment.)

In neutral, standard writing, your sentence is more natural without the comma.

Why is the present tense (sienten, tienen, piensan) used here instead of a future tense for la próxima crisis?

Spanish present tense can express:

current actions:
- Sienten estrés ahora. – They feel stress now.
habitual/generic situations:
- Muchos adultos sienten estrés… – Many adults feel stress (as a general fact).

Even though la próxima crisis económica refers to something in the future, the thinking and stress are happening now, so we use present:

piensan en la próxima crisis económica
they think about the next economic crisis (now / habitually)

We would only use future tense if the action itself is in the future:

Pensarán en la próxima crisis económica.
They will think about the next economic crisis (later).

So the present tense here signals a current, ongoing, or habitual situation.

Why is estrés written with an accent, and is it always singular?

The accent in estrés marks the stressed syllable: es‑trés.
Without the accent, by default rules, we might expect stress on es (the second‑to‑last syllable), which would be wrong.
Estrés is usually treated like an uncountable noun in Spanish, similar to English “stress”:
- Siento mucho estrés. – I feel a lot of stress.

There is a plural estreses, but it’s rare and sounds technical or forced. In almost all everyday contexts, you’ll only use estrés in the singular, even if the person is under “many stresses” or various sources of stress.