Learn Hardcore Croatian: Volim kad vidim da mladi volontiraju i sudjeluju u životu grada. - I like it when I see that young people volunteer and participate in the life of the city.

Kad is shorter and more common in everyday spoken Croatian.
Kada sounds a bit more formal or careful, and is very common in written language and careful speech.

In this sentence you could say either:

Volim kad vidim...
Volim kada vidim...

Both are correct; the meaning does not change. The choice is mostly about style and formality, not grammar.

Why is there da in vidim da mladi volontiraju? What does this da do?

Here da introduces a subordinate clause, roughly like that in English:

vidim da mladi volontiraju ≈ I see that young people volunteer

So the structure is:

vidim – I see
da mladi volontiraju i sudjeluju u životu grada – that young people volunteer and participate in the life of the city

In Croatian, da + full clause is very common after verbs of perception, thought, saying, etc.:

Znam da je teško. – I know that it’s hard.
Čujem da dolaziš sutra. – I hear that you’re coming tomorrow.

You cannot just drop da and keep the same word order; vidim mladi volontiraju is incorrect. Another option, with a slightly different structure, would be:

Vidim mlade kako volontiraju. – I see young people volunteering.

That uses kako instead of da and an object (mlade) instead of a full da-clause.

Why is it mladi and not mlade or mladih? What exactly does mladi mean here?

Mladi here is an adjective used as a noun, meaning young people.

Grammatically:

mladi is masculine plural nominative of the adjective mlad (young).
When used by itself like this, mladi means young people (in general).

So:

mladi volontiraju = (the) young (people) volunteer

Why not the others?

mlade – could be feminine plural nominative (young women) or accusative plural; not what is meant here.
mladih – genitive plural (of young people), wrong case for the subject of the verb.

Because mladi is the subject of volontiraju / sudjeluju, it must be in nominative plural: mladi.

Could we say mladi ljudi instead of just mladi? Is there any difference?

Yes, you can say both:

mladi – young people (slightly more general/abstract)
mladi ljudi – young people (literally young people/young persons)

In this sentence:

Volim kad vidim da mladi volontiraju...
Volim kad vidim da mladi ljudi volontiraju...

Both are correct. Mladi ljudi sounds a bit more explicit and concrete. Mladi alone is very common in public speech, media, and everyday language when talking about young people as a social group.

Why are the verbs volontiraju and sudjeluju in the 3rd person plural? Could they be in another form?

They agree with the subject mladi (young people), which is plural:

mladi volontiraju – young people volunteer
mladi sudjeluju – young people participate

So you need 3rd person plural:

oni volontiraju – they volunteer
oni sudjeluju – they participate

You could not change them to singular (volontira, sudjeluje) unless you changed the subject to a singular noun or pronoun. The tense is present tense, describing a general or repeated situation (young people in general tend to do this), not one specific event.

Why is it u životu grada and not something like u život grada or u život grada?

U životu grada uses the locative case after the preposition u with a static meaning (in / within something):

u (čemu?) životu – in the life (locative singular of život)
koga/čega? grada – of the city (genitive singular of grad)

So:

u životu grada = in the life of the city

Why not the others?

u život grada – wrong case; u + accusative would mean motion into something (into the life of the city), but you still need the preposition u twice or rephrase, and it would sound odd here.
u život grada without changing život to životu is simply not the correct locative form.

The pattern is common:

u životu škole – in the life of the school
u životu zajednice – in the life of the community

What is the difference between u životu grada and simply u gradu?

They express different ideas:

u gradu – in the city (physical space or location)
u životu grada – in the life of the city (its social, cultural, public life)

In sudjeluju u životu grada, the focus is on being active in events, organizations, cultural life, politics, etc., not just physically being present in the city. Saying sudjeluju u gradu would sound incomplete or strange; you almost always need an activity or sphere: u školskim aktivnostima, u politici, u društvu etc.

Can I move the parts of the sentence around, like starting with Kad vidim or moving volontiraju and sudjeluju?

Yes, Croatian word order is relatively flexible, especially for emphasis. Some natural alternatives:

Kad vidim da mladi volontiraju i sudjeluju u životu grada, volim to.
Volim kad vidim da mladi sudjeluju u životu grada i volontiraju.

The core order inside the da-clause should still be:

subject mladi before the verbs (very normal)
verbs directly after the subject
prepositional phrase u životu grada usually after the verbs

Something like Volim kad vidim da volontiraju mladi u životu grada is possible but sounds marked and emphasizes volontiraju or mladi in an unusual way. For a learner, it’s best to keep the original order.

Why is it Volim kad vidim... and not something like Sviđa mi se kad vidim...? Are volim and sviđa mi se different?

Both translate as I like, but they behave differently:

voljeti (volim) – literally to love, but often used as to like
- activity or situation:
- Volim kad vidim da... – I like it when I see that...
- Volim čitati. – I like reading.
sviđati se (impersonal reflexive verb):
- Sviđa mi se taj grad. – I like that city. (literally: That city pleases me.)

You could say:

Sviđa mi se kad vidim da mladi volontiraju...

It’s grammatical and natural; it just slightly changes the focus:

Volim kad vidim... – more active, more personal preference.
Sviđa mi se kad vidim... – more like This pleases me / I find this nice.

Why is vidim present tense? In English I might say “I like it when I see...” or “I like seeing...”. Could Croatian use another form?

Present tense vidim works like English when I see:

Volim kad vidim... – I like (it) when I see...

This describes something general and repeated: whenever you happen to see it, you like it.

Other options are possible but change the structure:

Volim vidjeti da mladi volontiraju... – I like to see that young people volunteer...
Volim gledati mlade kako volontiraju... – I like watching young people volunteering...

The original kad vidim emphasizes the moment of noticing/realizing, not the activity of watching.

What’s the difference between volontirati and saying biti volonter?

Both relate to volunteering, but they’re used differently:

volontirati – verb: to volunteer / to do volunteer work
- Mladi volontiraju. – Young people volunteer.
- Volontiram u udruzi. – I volunteer in an association.
biti volonter – to be a volunteer (noun + verb biti):
- Mladi su volonteri. – Young people are volunteers.
- On je volonter u školi. – He is a volunteer at the school.

In your sentence, we’re talking about the action, so volontiraju is the natural choice.

Is sudjeluju an irregular form? How is it related to the infinitive sudjelovati?

Sudjeluju is the regular 3rd person plural present tense of sudjelovati (to participate):

infinitive: sudjelovati
1st person sg: sudjelujem
2nd person sg: sudjeluješ
3rd person sg: sudjeluje
1st person pl: sudjelujemo
2nd person pl: sudjelujete
3rd person pl: sudjeluju

So:

Mladi sudjeluju u životu grada. – Young people participate in the life of the city.

It looks irregular only because the infinitive ends in -ovati, but the present tense uses the stem sudjeluj-. This is a normal pattern for many -ovati verbs (e.g. putovati → putuju, nastupovati → nastupuje).