Sy ryg die soom en steek dit vas.

Yes. Sy can mean either:

• she as a subject pronoun, or
• his as a possessive word

In this sentence, Sy ryg die soom ..., sy is the subject, so it means she.

A quick comparison:

• Sy ryg die soom. = She bastes/threads the hem.
• sy rok = his dress

Afrikaans does not capitalize these pronouns unless they begin the sentence, so you work it out from the sentence structure.

Die here means the.

So:

• die soom = the hem

Afrikaans uses die for the with both singular and plural nouns:

• die boek = the book
• die boeke = the books

So unlike English, there is no separate word for singular the and plural the.

Soom means hem, especially in sewing or clothing.

So die soom is the hem of a skirt, dress, trousers, and so on.

This sentence is clearly from a sewing context, so that helps explain the verbs too.

In a sewing context, ryg usually means something like:

• to baste
• to tack
• to make a loose running stitch
• sometimes more literally to thread/string

In this sentence, it is probably being used in the sewing sense: she is working the hem with a temporary or preparatory stitch before fastening it.

So it is not the everyday English verb to rig. It is a sewing word here.

Steek ... vas is a separable verb expression. In this context it means something like:

• fasten ... down
• stitch ... in place
• pin ... down, depending on context

The important grammar point is that the idea belongs together:

• vassteek = to fasten/stitch down

But in a normal main clause, Afrikaans often splits separable verbs:

• Sy steek dit vas. = She fastens/stitches it down.

So steek appears earlier, and vas moves to the end.

Because vassteek is acting like a separable verb.

Afrikaans often does this with verbs made up of:

• a verb + particle/adverb/preposition-like element

Examples:

• aantrek → Ek trek my jas aan.
• opstaan → Hy staan vroeg op.
• vassteek → Sy steek dit vas.

So in your sentence:

• steek is the verb part
• vas is the particle that completes the meaning

That is why vas comes at the end of the clause.

Dit means it and refers back to die soom.

So:

• Sy ryg die soom en steek dit vas. = She bastes the hem and fastens it down.

Afrikaans uses dit just like English uses it to avoid repeating the noun.

Afrikaans nouns do not have the same kind of grammatical gender system that languages like German or Dutch have.

In practice, for things and objects, Afrikaans commonly uses dit = it.

So:

• die soom → dit

That is normal and natural here because a hem is an object, not a person.

Afrikaans verbs usually do not change form according to the subject in the present tense.

So the verb stays the same:

• Ek ryg = I baste
• Jy ryg = you baste
• Sy ryg = she bastes
• Hulle ryg = they baste

And likewise:

• Ek steek
• Sy steek
• Hulle steek

This is much simpler than English, where we say I stitch but she stitches.

Because Afrikaans, like English, can leave out the repeated subject when the same subject continues across two coordinated verbs.

So:

• Sy ryg die soom en steek dit vas.

means:

• She bastes the hem and fastens it down.

You could think of it as short for:

• Sy ryg die soom en sy steek dit vas.

But repeating sy would usually sound unnecessary here.

Yes, it is the same basic verb, and that is something learners often notice.

Steek has a wider range of meanings, including things like:

• stab
• pierce
• insert
• stitch
• put/push into
• in combinations like vassteek, fasten down

Many Afrikaans verbs cover a wider range of meanings than one exact English verb does, so you have to learn them by context.

In a sewing sentence like this, steek is not about stabbing someone; it is about stitching or fastening fabric.

The basic structure is:

• Sy = subject
• ryg = first verb
• die soom = object
• en = and
• steek = second verb
• dit = object
• vas = separable particle

So:

Subject + verb + object + and + verb + object + particle

That is a very normal Afrikaans pattern for a main clause.

Good question, because this helps show the separable verb pattern.

In a main clause:

• Sy steek dit vas.

But where the infinitive or verb cluster appears at the end, the parts often stay together:

• Sy wil dit vassteek. = She wants to fasten/stitch it down.
• ..., omdat sy dit wil vassteek. = ..., because she wants to fasten/stitch it down.

So learners often need to recognize both:

• steek ... vas
• vassteek

They are the same verb idea in different sentence structures.