Ona radi sporije, jer je umorna.

Yes. Serbian often drops subject pronouns when the verb already shows who the subject is.

So:

• Ona radi sporije, jer je umorna.
• Radi sporije, jer je umorna.

Both are correct.

Using ona can add emphasis, contrast, or clarity. For example, it may mean she works more slowly, not someone else.

Radi is the 3rd person singular present tense of raditi.

It can mean:

• she works
• she is working
• sometimes it works / it is functioning, depending on context

In this sentence, it means she works / she is working more slowly.

Because sporije is the comparative adverb form, meaning more slowly or slower in English.

Compare:

• sporo = slowly
• sporije = more slowly

So:

• Ona radi sporo. = She works slowly.
• Ona radi sporije. = She works more slowly.

English often uses slower where Serbian uses the comparative adverb sporije.

Here it is an adverb, because it describes how she works.

• spor = slow (adjective, masculine)
• spora = slow (adjective, feminine)
• sporo = slowly / slow (adverb or neuter adjective form, depending on context)
• sporije = more slowly

Since it modifies the verb radi, it is functioning as an adverb.

Here je means is.

The phrase je umorna means is tired.

So:

• jer = because
• je = is
• umorna = tired

Together: jer je umorna = because she is tired

Because it describes ona, which means she.

In Serbian, adjectives agree with the person or thing they describe in gender, number, and often case.

So:

• umoran = tired (masculine)
• umorna = tired (feminine)
• umorno = tired (neuter)

Since the subject is she, the adjective must be feminine: umorna.

Yes, you can.

• jer je umorna = because she is tired
• jer je ona umorna = because she is tired

The version with ona is more explicit or emphatic. Usually Serbian prefers the shorter version unless there is a reason to stress she.

Because jer introduces a subordinate clause, like because in English.

So the sentence is divided into:

• main clause: Ona radi sporije
• subordinate clause: jer je umorna

In standard Serbian writing, a comma is normally used before jer.

Yes.

You can say:

• Ona radi sporije, jer je umorna.
• Ona radi sporije zato što je umorna.

Both mean the same thing: She works more slowly because she is tired.

Very roughly:

• jer is a very common neutral word for because
• zato što also means because, and can sometimes feel a bit more explicit or slightly heavier in style

By itself, sporije just means more slowly. The comparison is not stated directly.

It could mean:

• more slowly than before
• more slowly than usual
• more slowly than someone else
• more slowly than expected

The exact comparison depends on context.

It can mean either, depending on context.

Raditi is a broad verb. It can mean:

• to work
• to do
• to function

So Ona radi sporije could mean:

• She is working more slowly.
• She does things more slowly.
• She is performing her task more slowly.

Context tells you which reading is intended.

You would change the pronoun and the adjective:

• On radi sporije, jer je umoran.

Changes:

• ona → on
• umorna → umoran

The verb radi stays the same, because both he and she use the same 3rd person singular verb form in Serbian.

A simple pronunciation guide:

• Ona ≈ OH-nah
• radi ≈ RAH-dee
• sporije ≈ SPOR-ee-yeh
• jer ≈ yer
• je ≈ yeh
• umorna ≈ oo-MOR-nah

A rough full pronunciation:

OH-nah RAH-dee SPOR-ee-yeh, yer yeh oo-MOR-nah.

This is only an approximation for English speakers, but it is a useful start.