Ese voluntario es muy solidario: siempre ayuda a quien lo necesita sin pedir nada a cambio.

Both este and ese mean this/that, but they point to different degrees of distance.

• este voluntario = this volunteer (closer to the speaker, physically or mentally)
• ese voluntario = that volunteer (a bit farther away, or already identified in the conversation)

In real Spanish, the difference is often about how the speaker is framing the person, not just literal physical distance. Here, ese voluntario sounds like that volunteer / that particular volunteer.

Here, voluntario is a noun: a volunteer.

Spanish often uses the same word as both a noun and an adjective:

• Es voluntario. = He is a volunteer. / He does it voluntarily.
  (This can be ambiguous depending on context.)
• Ese voluntario = That volunteer.
  (Clearly a noun here.)

In this sentence, ese makes it obvious that voluntario is being used as a noun.

Because solidario is describing a character trait or general quality, so Spanish uses ser, not estar.

• es muy solidario = he is very caring/supportive/generous by nature
• está would usually suggest a temporary state, which does not fit well here

A native English speaker can think of it like this:

• ser = what someone is like
• estar = how someone is at the moment

Since the sentence is describing the volunteer’s usual character, es is the natural choice.

Solidario does not usually translate literally as English solidary in normal everyday speech. In this context, it means something like:

• caring
• supportive
• compassionate
• willing to help others
• sometimes generous in a social or human sense

So muy solidario suggests a person who shows concern for others and helps them.

The present tense is used here to describe a habit or something the person does regularly.

• siempre ayuda = he always helps

Spanish uses the simple present very often for habitual actions, just like English does:

• Siempre llega temprano. = He always arrives early.
• Siempre ayuda... = He always helps...

There is no need for a progressive form like está ayudando here, because the sentence is not about what he is doing right now, but about what he generally does.

That a is there because the verb ayudar normally goes with a before a person:

• ayudar a alguien = to help someone

So:

• ayuda a María
• ayuda a sus vecinos
• ayuda a quien lo necesita

Even though English says just help someone, Spanish usually says ayudar a alguien.

Here, quien means whoever, the person who, or those who, depending on how naturally you translate it.

So:

• a quien lo necesita = whoever needs it/him or the person who needs it/him

In Spanish, quien is often used after a preposition like a to refer to a person in a general way.

You could think of the whole phrase as:

• ayuda a quien lo necesita = he helps anyone who needs it/him

Spanish often uses quien in a generic way, even when the meaning can include more than one person.

So a quien lo necesita can mean:

• anyone who needs it/him
• whoever needs it/him

If you wanted to make the plural idea more explicit, you might also hear:

• a quienes lo necesitan

But the singular quien is very common when the idea is general and indefinite.

Lo is a direct object pronoun. It stands in for something already understood from context.

In this sentence, it most naturally refers to the help being given, or more loosely to the volunteer’s help/support.

So:

• quien lo necesita = whoever needs it

In context, it is basically his help or help in general.

Spanish often uses object pronouns this way even when English might leave the object more vague.

Because necesitar takes a direct object, not an indirect one.

• necesitar algo = to need something
• necesitar a alguien = to need someone

So the pronouns are:

• lo / la / los / las for direct objects

That is why lo necesita is standard here.

After sin (without), Spanish normally uses an infinitive when the subject is the same as the main clause.

• sin pedir nada a cambio = without asking for anything in return

This is very natural Spanish.

Compare:

• Trabaja sin descansar. = He works without resting.
• Se fue sin decir nada. = He left without saying anything.

So here, sin pedir is the normal structure.

Nada a cambio is a very common expression meaning:

• nothing in return
• anything in exchange

So:

• sin pedir nada a cambio = without asking for anything in return

You can learn a cambio as a fixed phrase meaning in exchange.

Examples:

• Me ayudó sin pedir nada a cambio.
• Te doy esto a cambio de tu ayuda. = I give you this in exchange for your help.

The colon introduces an explanation or justification of the previous statement.

• Ese voluntario es muy solidario:
  = That volunteer is very caring/supportive:
• siempre ayuda...
  = because he always helps...

So the second part explains why the speaker says he is muy solidario.

In English, you might use either a colon or just a full stop/comma depending on style.

Yes. That would also be correct:

• Ese voluntario siempre ayuda a quien lo necesita sin pedir nada a cambio.

The version with es muy solidario: adds an explicit judgment first, and then explains it. It gives the sentence a more descriptive, slightly more emphatic tone.

So:

• Es muy solidario: siempre ayuda...
  = first the opinion, then the proof
• Siempre ayuda...
  = just the action, without that extra comment

Not in natural English. Word-for-word, that is close to the Spanish structure, but natural English would say:

• whoever needs it
• anyone who needs it
• the person who needs it

This is a good example of a phrase that is best understood as a whole rather than translated piece by piece.