Сначала сметайте шов, потом застрочите его.

Because they are imperative forms: commands or instructions.

The ending -йте is used for:

• plural you: speaking to more than one person
• polite singular you: speaking formally to one person

So сметайте and застрочите mean something like baste / machine-stitch as instructions.

The informal singular forms would be:

• сметай
• застрочи

They are perfective imperative forms.

• сметайте = imperative of сметать in the sewing sense
• застрочите = imperative of застрочить

In Russian instructions, perfective imperatives are very common when the speaker means do this action once and complete it, then move on to the next step.

That fits this sentence well:

• first complete the basting
• then complete the machine stitching

Because the sentence gives a sequence of completed steps.

Russian often uses the perfective imperative for step-by-step instructions when each action is meant as a whole:

• Сначала сметайте... = first baste it
• потом застрочите... = then stitch it

If imperfective imperatives were used instead, the tone would often sound more like:

• focus on the process
• repeated/habitual action
• general manner of doing something

Here the speaker is clearly talking about one finished action after another.

No. This is a very important point.

In this sentence, сметать is a sewing verb, meaning to baste or to tack loosely by hand before final stitching.

Russian also has another сметать meaning to sweep away / brush away, but that is a different lexical item for practical purposes.

So in sewing:

• сметать шов = to baste a seam

Do not interpret it here as anything related to sweeping.

In sewing, застрочить means to stitch, usually by machine, often as the final sewn line.

So the contrast is roughly:

• сметать = baste temporarily, usually with loose hand stitches
• застрочить = stitch it properly, usually on a sewing machine

That is why the order is:

1. baste first
2. then machine-stitch

Because it is the direct object of сметайте, and for a masculine inanimate noun like шов, the accusative singular is the same as the nominative singular.

So:

• nominative: шов
• accusative: шов

That is normal Russian grammar.

Because after застрочите you need a direct object, so Russian uses the accusative form of the pronoun.

• он = he / it, nominative
• его = him / it, accusative or genitive

Here его means it, referring back to шов.

So:

• застрочите его = stitch it

Yes, it could be omitted if the object is obvious from context:

• Сначала сметайте шов, потом застрочите.

That can work, especially in practical instructions.

But keeping его is very natural because it makes the reference explicit:

• first baste the seam
• then stitch it

So the version with его is clear and perfectly normal.

Because сначала ... потом ... already shows the sequence very clearly.

Russian often links instructions like this without needing и:

• Сначала ... , потом ...

This is a very natural pattern for procedural language:

• First do X, then do Y.

Adding и is not necessary.

They are adverbs marking order in time.

• сначала = first, at first
• потом = then, afterward

They help structure the instruction into two stages.

This is a very common Russian pattern:

• Сначала A, потом B.
• First A, then B.

No, Russian word order is fairly flexible, but the given order is the most neutral for an instruction:

• Сначала сметайте шов, потом застрочите его.

It starts with the time-order words сначала and потом, which makes the procedure easy to follow.

You could rearrange it for emphasis, for example:

• Шов сначала сметайте, потом застрочите его.

But the original is the most straightforward and natural in a manual or spoken instruction.

Although it is spelled его, it is normally pronounced roughly like yivo.

So:

• spelling: его
• usual pronunciation: иво / йиво approximately

This is a very common feature of Russian pronouns and adjective endings with -ого / -его in standard pronunciation.