Learn Hardcore Croatian: Tramvaj je jutros bio bučan, ali sada su kafić i mali bar mirni. - The tram was noisy this morning, but now the cafe and the small bar are quiet.

If you say Tramvaj je bučan, that is present tense: The tram is noisy (now / generally).
Because jutros (this morning) refers to an earlier time today, Croatian prefers the past form je bio, not the present je.

Can jutros be placed in other positions in the sentence?

Yes. Adverbs of time like jutros are quite flexible in Croatian. All of these are grammatical:

Jutros je tramvaj bio bučan.
Tramvaj je jutros bio bučan. (very common, neutral)
Tramvaj je bio jutros bučan. (also possible, but sounds a bit less neutral)

Changing the position can slightly affect emphasis or style, but the basic meaning stays the same.

Why is it bio bučan and not bio bučno?

Because adjectives used with a specific noun must agree with that noun in gender and number.

tramvaj is masculine singular
the past participle bio is masculine singular
the adjective bučan is also masculine singular

bučno is the neuter form or an adverb (noisily / it is noisy in a general sense).
With a masculine noun like tramvaj, you need bučan, not bučno:

Tramvaj je bio bučan. ✅
Tramvaj je bio bučno. ❌ (ungrammatical here)

What is the role of ali in this sentence?

ali is a coordinating conjunction meaning but.

It introduces a contrast between the first and second clause:

Tramvaj je jutros bio bučan, ali sada su kafić i mali bar mirni.
= The tram was noisy this morning, but now the café and the small bar are quiet.

Why do we use su with kafić i mali bar?

su is the 3rd person plural present of biti (to be), used with plural subjects.

Although kafić and bar are each singular, together they form a compound subject, so grammatically they count as “they”:

kafić i mali bar = the café and the small bar → they
therefore: sada su kafić i mali bar mirni (now the café and the small bar are quiet)

Why is the adjective mirni plural?

Again, because of agreement. The subject of the second clause is:

kafić (masculine singular)
mali bar (masculine singular)
Together: masculine plural

So the predicate adjective must be masculine plural nominative: mirni.

Compare:

Kafić je miran. (one café → masculine singular: miran)
Kafić i bar su mirni. (two places → masculine plural: mirni)

Why is it mali bar and not malo bar?

Because bar is a masculine singular noun, so its adjective must match:

mali bar (masculine singular nominative) = small bar

malo can be:

neuter singular adjective (e.g. malo selo – a small village), or
an adverb/quantifier meaning a little, a bit.

With bar (masculine), you need mali, not malo.

Why are kafić and mali bar in the nominative case?

They are the subject of the second clause:

sada su kafić i mali bar mirni

Subjects in Croatian are in the nominative case.
If these nouns were, for example, objects or after prepositions, they would change:

Vidim kafić i bar. (accusative, same form here) – I see the café and the bar.
Sjedim u kafiću. (locative) – I’m sitting in the café.
Idem u bar. (accusative) – I’m going to the bar.

In the sentence you gave, since they are simply the things that are quiet, they stay in the nominative: kafić i mali bar.

Can we say sad instead of sada?

Yes. sad is a shorter, very common form of sada, and both mean now.

sada – a bit more formal or emphatic
sad – more colloquial, everyday speech

So you can also say:

Tramvaj je jutros bio bučan, ali sad su kafić i mali bar mirni.

Can we change the word order to ali kafić i mali bar su sada mirni?

Yes, that is also correct:

… ali sada su kafić i mali bar mirni.
… ali kafić i mali bar su sada mirni.

Croatian word order is relatively flexible. Both versions mean the same thing; the difference is in rhythm and slight emphasis:

sada su kafić i mali bar mirni – light emphasis on “now” (contrast with this morning)
kafić i mali bar su sada mirni – slightly more focus on the café and the bar as the topic

Why is there no word for “the” in tramvaj, kafić, or bar?

Croatian has no articles like English “a/an” or “the”.

tramvaj can mean a tram or the tram, depending on context.
kafić and mali bar can mean a café / the café, a small bar / the small bar.

Definiteness is usually understood from context, word order, or additional words (like taj = that, onaj = that (over there), ovaj = this, etc.).