Čekala sam ceo dan.

Because the past participle in Serbian agrees with the speaker’s gender and number.

• čekala = feminine singular
• čekao = masculine singular

So Čekala sam ceo dan. tells you the speaker is female.
A male speaker would say Čekao sam ceo dan.

sam is the 1st person singular present-tense form of biti (to be), and it is used as an auxiliary to form the Serbian past tense.

So:

• čekala = past participle
• sam = auxiliary am

Together, čekala sam means I waited / I was waiting.

Serbian often drops subject pronouns when they are not needed.

In this sentence, sam already shows first person singular (I), so ja is unnecessary.

• Čekala sam ceo dan. = normal, natural
• Ja sam čekala ceo dan. = also correct, but adds emphasis, contrast, or clarity

So Serbian does not need to say ja every time English uses I.

Because sam is a clitic, and Serbian clitics usually go in second position in the sentence or clause.

That leads to patterns like:

• Ja sam čekala ceo dan.
• Čekala sam ceo dan.

In a sentence without an expressed subject, starting with the participle and then placing sam after it is very natural.

It can correspond to both, depending on context.

The verb čekati is imperfective, so it emphasizes the process/duration of waiting rather than a single completed event. Because of that, English often translates it as:

• I was waiting all day
• or I waited all day

With ceo dan (the whole day / all day), the idea of duration is especially strong.

ceo dan means the whole day / all day.

• ceo = whole, entire
• dan = day

Here it is in the accusative, which Serbian often uses for duration of time.

A useful point: for many masculine inanimate nouns, the accusative looks the same as the nominative, so:

• nominative: ceo dan
• accusative: ceo dan

That is why the phrase looks unchanged.

Because Serbian čekati usually takes a direct object, not a preposition like English wait for.

For example:

• Čekam autobus. = I’m waiting for the bus.
• Čekam te. = I’m waiting for you.

So if you wanted to say who or what you were waiting for, Serbian would normally not use a separate word meaning for.

In Čekala sam ceo dan., the object is simply not stated.

Yes, absolutely.

• Čekala sam ceo dan. = neutral, natural
• Ja sam čekala ceo dan. = correct, but ja adds emphasis

You might use ja if you want contrast, for example:

• On nije čekao, ali ja sam čekala ceo dan.
  He didn’t wait, but I waited all day.

So the version without ja is usually the default.

Yes, in many varieties you can.

• ceo dan is the usual Ekavian form
• cijeli dan is an Ijekavian form

Both mean the whole day / all day.

If you are specifically learning standard Serbian as commonly written in Ekavian, ceo dan is the expected form here.

Because čekati is imperfective, and it fits an action lasting over a period of time, especially with ceo dan.

• čekati = to wait, to be waiting
• sačekati = perfective, more like wait until, wait for and reach an endpoint, or wait successfully

For simple duration, Čekala sam ceo dan. is the natural choice.

Using sačekati would change the nuance and would not be the normal way to express plain ongoing waiting for the whole day.

A rough English-friendly pronunciation is:

CHEH-kah-lah sahm TSEH-oh dahn

A few key sounds:

• Č sounds like ch in church
• c sounds like ts in cats
• j in Serbian sounds like English y

So:

• Čekala ≈ CHEH-kah-lah
• ceo ≈ TSEH-oh

That said, the exact stress is not shown in normal spelling.