Ti si juče kasno čekala autobus.

Yes. Serbian often omits subject pronouns when they are clear from the verb form or context.

So instead of:

Ti si juče kasno čekala autobus.

you could very often just say:

Juče si kasno čekala autobus.

Adding ti gives extra emphasis, contrast, or clarity, for example:

• Ja sam došla ranije, a ti si juče kasno čekala autobus.
• You, on the other hand, were waiting for the bus late yesterday.

So ti is not wrong at all, but it is often more emphatic than English you.

This is the past tense in Serbian, often called the perfect.

It is formed with:

• the present tense of biti = to be
• plus the past active participle

Here:

• si = you are / auxiliary part used for the past
• čekala = past participle of čekati

So:

• ti si čekala = you waited / you were waiting

This is a normal Serbian past-tense structure.

Compare:

• ja sam čekala = I waited
• ti si čekala = you waited
• on je čekao = he waited
• ona je čekala = she waited

Because in Serbian, the past participle agrees with the gender of the subject in the singular.

So:

• čekao = masculine singular
• čekala = feminine singular
• čekalo = neuter singular

That means this sentence is addressed to or refers to a female person:

• Ti si juče kasno čekala autobus. = said to a woman/girl

If you were speaking to a man, it would be:

• Ti si juče kasno čekao autobus.

This is something English does not do, so it often stands out to learners.

Yes. Čekati directly takes an object, so Serbian says literally:

• čekati autobus = wait for the bus

There is no preposition like English for here.

More examples:

• Čekam te. = I’m waiting for you.
• Čekamo voz. = We’re waiting for the train.
• Čekala je prijatelja. = She was waiting for her friend.

So autobus is the direct object of čekala.

It is in the accusative. The reason it looks unchanged is that for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: autobus
• accusative: autobus

That is completely normal.

Compare with an animate masculine noun:

• nominative: prijatelj = friend
• accusative: prijatelja

So:

• Čekala sam autobus. = I waited for the bus.
• Čekala sam prijatelja. = I waited for my friend.

Here kasno means late.

So juče kasno gives the idea of late yesterday or yesterday late depending on context, and in natural English this would usually be interpreted as something like:

• yesterday late
• late yesterday
• or in a smoother translation, you were waiting for the bus late yesterday

Kasno can also appear in other expressions:

• Došla je kasno. = She arrived late.
• Kasno je. = It’s late.

So in this sentence, kasno is an adverb describing the time of the waiting.

Yes, Serbian word order is fairly flexible, but not completely free.

In this sentence:

• juče = yesterday
• kasno = late

So juče kasno is a natural sequence meaning late yesterday.

You can also hear variations depending on emphasis, for example:

• Juče si kasno čekala autobus.
• Kasno si juče čekala autobus.

But not every variation sounds equally natural in every context. Serbian often moves words around for focus or emphasis.

The version you were given is natural, especially if ti is being emphasized.

Normally, si is a clitic, and clitics tend to go in the second position in the sentence or clause.

That is why:

• Ti si juče kasno čekala autobus. is natural

but:

• Ti juče si kasno čekala autobus. sounds wrong or unnatural in standard Serbian

If the sentence starts with juče, then si usually comes right after it:

• Juče si kasno čekala autobus.

This second-position rule is very important in Serbian and often takes practice.

It can often correspond to either, depending on context.

Serbian past tense here does not force exactly the same distinction that English makes between:

• you waited
• you were waiting

The verb čekati is imperfective, which often fits ongoing, repeated, or uncompleted action, but in the past tense sentence itself, English may translate it in different ways depending on context.

So this sentence could be understood as:

• You waited for the bus late yesterday
• You were waiting for the bus late yesterday

If you want the best English version, context decides.

Čekati is the normal imperfective verb meaning to wait.

Imperfective verbs are commonly used for:

• ongoing actions
• habitual actions
• background situations
• actions viewed as a process

That fits waiting very well, because waiting is often seen as a duration or process.

A related perfective verb is sačekati, which means more like wait until, wait for and complete the waiting, or wait for someone/something to arrive in a bounded sense.

Compare:

• Čekala sam autobus. = I was waiting for the bus / I waited for the bus.
• Sačekala sam autobus. = I waited for the bus and successfully stayed until it came, depending on context.

In many ordinary situations, čekati is the expected verb.

Yes. Each adverb adds a separate piece of time information.

• Ti si čekala autobus. = You were waiting for the bus.
• Ti si juče čekala autobus. = You were waiting for the bus yesterday.
• Ti si kasno čekala autobus. = You were waiting for the bus late.

With both together:

• Ti si juče kasno čekala autobus. = You were waiting for the bus late yesterday.

So juče tells which day, and kasno tells what part of the time frame.

Because Serbian has no articles like English a and the.

So autobus can mean:

• a bus
• the bus

The exact meaning depends on context.

For example:

• Čekala sam autobus. could mean either I was waiting for a bus or I was waiting for the bus.

Serbian uses context, word order, and sometimes other expressions to make the meaning clear when needed.

You would change the past participle so that it agrees with the subject.

To a man:

• Ti si juče kasno čekao autobus.

To a woman:

• Ti si juče kasno čekala autobus.

To a group that includes at least one male, or a mixed group:

• Vi ste juče kasno čekali autobus.

To a group of females:

• Vi ste juče kasno čekale autobus.

Also note that ti si becomes vi ste for plural or formal you.

Yes. Vi can be:

• plural you
• or formal singular you

So if you were politely addressing one woman, you could say:

• Vi ste juče kasno čekali autobus.

In standard polite usage, the participle is often plural in form with formal Vi, even when speaking to one person.

So this is an important social distinction:

• ti = informal singular
• vi = plural or formal singular

The dictionary form is čekati.

When learning Serbian verbs, you usually memorize the infinitive, such as:

• čekati = to wait
• doći = to come
• videti = to see

From čekati, you get forms like:

• čekam = I wait / I am waiting
• čekaš = you wait
• čekao / čekala = waited
• čekajući = waiting

So if you look up čekala in a dictionary, you should search for čekati.