Danas sam čupala korov gotovo sat vremena, pa me sada bole leđa.

It is the past tense most commonly used in everyday Croatian: the perfect.

It is built with:

• the auxiliary sam = am
• the past participle čupala

So sam čupala literally looks like am pulled/pulling, but in natural English it can correspond to:

• I pulled
• I was pulling
• I have been pulling

The exact English translation depends on context. In this sentence, because of gotovo sat vremena, it clearly describes an activity that lasted for a while in the past.

Because the past participle agrees with the gender and number of the subject.

Here the speaker is:

• first person singular
• female

So the form is čupala.

Compare:

• sam čupala = I was pulling/weeding (female speaker)
• sam čupao = I was pulling/weeding (male speaker)
• smo čupali = we were pulling/weeding (mixed or masculine plural)
• smo čupale = we were pulling/weeding (all-female plural)

This is a very common feature of Croatian past-tense forms.

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

So:

• Danas sam čupala... = natural
• Ja sam danas čupala... = also natural
• Sam danas čupala... = not acceptable

In other words, sam usually cannot stand freely at the beginning. It wants to come right after the first stressed word or phrase.

Yes. Čupati is imperfective.

That matters because the sentence describes:

• an activity in progress
• something that lasted for a period of time
• the process, not just the result

The phrase gotovo sat vremena fits very naturally with an imperfective verb.

A perfective verb would shift the focus toward completion, for example:

• počupati
• iščupati

Those can suggest pulling out completely or finishing the job, while čupati simply describes the ongoing action.

So sam čupala korov gotovo sat vremena sounds like:

• I spent almost an hour pulling weeds rather than
• I finished pulling out the weeds

Because korov is often used as a collective or mass noun in Croatian.

So čupati korov means:

• pull weeds
• weed
• pull out the weedy growth

Even though English often uses the plural weeds, Croatian can use singular korov to refer to weed growth in general.

Also, grammatically:

• korov is the direct object
• here it is in the accusative singular
• for a masculine inanimate noun, the accusative singular is often the same as the nominative

So the form stays korov.

This is a very common Croatian way to express duration.

• sat = hour
• vremena = of time

So sat vremena literally means an hour of time, but naturally it just means an hour in a duration sense.

Examples:

• pet minuta = five minutes
• sat vremena = an hour
• dva sata = two hours
• tri dana = three days

Here:

• gotovo sat vremena = almost an hour

The word vremena is in the genitive, which is normal in this type of time expression.

Gotovo means almost or nearly.

It modifies the time expression:

• gotovo sat vremena = almost an hour

So it tells you the action lasted just under an hour.

You could also see skoro in similar contexts:

• skoro sat vremena

Both are common, though gotovo can sound a bit more neutral or formal depending on context.

Here pa links the two clauses and shows a result or consequence.

So in this sentence it means something like:

• so
• and so
• which is why

Structure:

• I was pulling weeds for almost an hour
• so now my back hurts

It is not exactly the same as i (and) and not the same as jer (because).

Compare:

• ..., pa me sada bole leđa. = ..., so now my back hurts.
• ... jer me sada bole leđa. = ... because my back hurts now.
  That would reverse the logic and sound wrong for this meaning.

Because Croatian uses the verb boljeti in a structure where the person feeling pain is often expressed with an accusative clitic.

So:

• bole me leđa = my back hurts / my back is hurting me
• boli me glava = my head hurts
• bole me noge = my legs hurt

Here:

• me = me in the accusative
• leđa is the grammatical subject

So the literal structure is closer to:

• the back hurts me

Not:

• I hurt
• and not normally mi in standard usage here

Because leđa is grammatically plural.

Even though in English back is singular, Croatian leđa is a plural-only noun (pluralia tantum).

That means:

• leđa = back
• but grammatically it behaves like a plural noun

So the verb must also be plural:

• leđa bole not
• leđa boli

This is the same pattern as:

• Bole me leđa.
• Leđa me bole.

Both are correct; the difference is mainly emphasis and word order.

Yes. In Croatian, when pa connects two full clauses, a comma is normally used.

Here the two clauses are:

1. Danas sam čupala korov gotovo sat vremena
2. pa me sada bole leđa

Each clause has its own verb:

• sam čupala
• bole

That is why the comma is natural and standard here.

Croatian word order is fairly flexible, but not completely free. You can move things around for emphasis, especially adverbs and objects.

For example, these are all possible with slightly different focus:

• Danas sam čupala korov gotovo sat vremena, pa me sada bole leđa.
• Danas sam gotovo sat vremena čupala korov, pa me sada bole leđa.
• Korov sam danas čupala gotovo sat vremena, pa me sada bole leđa.
• Sada me bole leđa, jer sam danas čupala korov gotovo sat vremena.

What usually stays important is:

• clitics like sam and me follow clitic-placement rules
• the intended emphasis changes with word order

So the original sentence is very natural, but it is not the only possible arrangement.