Sy werk is swak.

In this sentence, sy is most naturally the possessive his.

So Sy werk is swak normally means His work is poor / weak.

This is a common point of confusion because sy can also mean she as a subject pronoun. Compare:

• Sy werk is swak. = His work is poor.
• Sy werk hard. = She works hard.

If you want to say Her work is poor, standard Afrikaans is:

• Haar werk is swak.

Here, werk is a noun, meaning work.

You can tell because the structure is:

• sy werk = his work
• is swak = is poor

So the sentence is built like:

• [possessive + noun] + [is] + [adjective]

Compare that with a sentence where werk is a verb:

• Sy werk swak. = She works poorly.

That sentence has no is, because werk there is the main verb.

Swak literally means weak, but in a sentence about someone's work it often means:

• poor
• bad
• not very good
• low in quality

So in this context, swak is usually best understood as poor rather than physically weak.

Is is the present-tense form of to be, just like English is.

It links the subject part (sy werk) to the description (swak):

• Sy werk = his work
• is = is
• swak = poor

So the sentence works exactly like English:

• His work is poor.

Without is, the sentence would mean something different if it were grammatical at all.

Because sy already makes the noun definite enough by showing possession.

• sy werk = his work

English works the same way:

• We say his work, not the his work
• We say her book, not the her book

So no article is needed.

Because it comes after is as a predicate adjective.

In Afrikaans, adjectives used after a linking verb like is usually stay in their basic form:

• Die werk is swak.
• Sy werk is swak.

So swak does not need to agree with the noun the way adjectives often do in some other languages.

It can, depending on context, because werk can refer to:

• work in general
• someone's work output
• sometimes a job

But in Sy werk is swak, the most natural meaning is that the quality of the person's work is poor.

If you were specifically talking about someone's employment, context would usually make that clear.

You would say:

• Sy werk swak.

That is different from:

• Sy werk is swak. = His work is poor.

This contrast is very useful:

• Sy werk swak. → sy = she, werk = verb
• Sy werk is swak. → sy = his, werk = noun

So the presence of is helps show that werk is a noun in your original sentence.

You would normally say:

• Haar werk is swak.

That is the standard way to express her work.

So the three useful patterns are:

• Sy werk is swak. = His work is poor.
• Haar werk is swak. = Her work is poor.
• Sy werk swak. = She works poorly.