Jučer sam bila tužna, ali danas sam sretna.

Croatian often drops subject pronouns when they are already clear from the verb form.

So instead of saying Ja sam bila tužna, Croatian usually just says Sam bila tužna? No—because sam is a clitic and needs proper placement, so the normal full sentence is Jučer sam bila tužna.

The verb form sam already tells you the subject is I. If you add ja, it usually gives extra emphasis:

• Ja sam bila tužna = I was sad / I was sad

So in neutral speech, leaving out ja is normal.

Those forms are feminine singular. This sentence is being said by a female speaker.

• bila = feminine form of been / was
• tužna = sad (feminine)
• sretna = happy (feminine)

If a man said the same sentence, it would be:

• Jučer sam bio tužan, ali danas sam sretan.

So Croatian adjectives, and also the past participle here, must agree with the gender of the speaker.

Sam is the 1st person singular present tense of biti (to be), meaning am.

In this sentence it appears twice:

• sam bila = I was
• sam sretna = I am happy

So:

• in the present, sam can mean am
• in the past, sam works as an auxiliary verb together with bila/bio

Croatian normally forms the past tense with:

present of biti + past participle

So:

• sam bila = I was (female speaker)
• sam bio = I was (male speaker)

This is the usual Croatian perfect tense, and it is very common for talking about past actions or states.

So English was becomes two words in Croatian:

• sam bila / sam bio

Because sam is a clitic. Croatian clitics usually go in the second position of the clause.

So:

• Jučer sam bila tužna
• Danas sam sretna

Here the first word is:

• Jučer = yesterday
• Danas = today

and the clitic sam comes right after it.

This second-position rule is very important in Croatian word order.

Yes, Croatian word order is fairly flexible, but not completely free. The clitic sam still has to obey its placement rules.

You can say, for example:

• Jučer sam bila tužna, ali danas sam sretna.
• Bila sam jučer tužna, ali danas sam sretna.

Both are possible, though the first one is probably the most neutral here.

What you generally cannot do is place sam just anywhere. For example, learners often make mistakes with clitics.

So yes, word order can change for focus or style, but sam needs careful placement.

Because ali means but, and it joins two contrasting clauses:

• Jučer sam bila tužna
• danas sam sretna

In Croatian, as in English, a comma is normally used before ali in this kind of sentence.

So:

• ..., ali ... = ..., but ...

They are adverbs of time:

• jučer = yesterday
• danas = today

In this use, they do not change for case, gender, or number.

That makes them easier to use than many Croatian nouns and adjectives. You can often place them at the beginning of the sentence for time emphasis, as in this example.

Because sam alone is already the correct present-tense form of to be for I am.

So:

• sam sretna = I am happy

There is no need for another verb. Croatian simply uses:

subject (often omitted) + sam + adjective

Examples:

• (Ja) sam umorna. = I am tired.
• (Ja) sam sretna. = I am happy.

A male speaker would use masculine forms:

• Jučer sam bio tužan, ali danas sam sretan.

Changes:

• bila → bio
• tužna → tužan
• sretna → sretan

The words jučer, sam, ali, and danas stay the same.

A simple pronunciation guide is:

• Jučer ≈ YOO-cher
• sam ≈ sahm
• bila ≈ BEE-lah
• tužna ≈ TOO-zhnah
• ali ≈ AH-lee
• danas ≈ DAH-nahs
• sretna ≈ SRET-nah

A few useful sound notes:

• j is pronounced like English y
• č sounds roughly like ch
• ž sounds like the s in measure

So the whole sentence is approximately:

YOO-cher sahm BEE-lah TOO-zhnah, AH-lee DAH-nahs sahm SRET-nah.