Портной распорол старый шов.

Портной is a noun meaning tailor, but it declines like an adjective. That is why its nominative singular masculine form ends in -ой.

Its case forms are:

• портной — nominative
• портного — genitive / accusative animate
• портному — dative
• портным — instrumental
• портном — prepositional

In this sentence, портной is the subject, so it is in the nominative singular.

Распорол is the past tense masculine singular form of the perfective verb распороть.

In Russian past tense, the verb agrees with the subject in gender and number:

• портной распорол — masculine singular
• портниха распорола — feminine singular
• ателье распороло — neuter singular
• портные распороли — plural

Since портной is masculine singular, распорол is the correct form.

Распороть is perfective. It presents the action as a completed result: the seam was successfully opened/unpicked.

That is why распорол fits a one-time finished action in the past.

If you wanted to emphasize the process or an ongoing/repeated action, you would use an imperfective verb, for example распарывать:

• Портной распарывал шов. — The tailor was unpicking the seam / used to unpick the seam.

The prefix рас- adds the sense of apart, open, or undoing.

So распороть means something like:

• open up by ripping
• unpick
• undo a seam

The spelling рас- instead of раз- happens because the next consonant, п, is voiceless. Russian commonly writes:

• раз- before voiced sounds
• рас- before voiceless sounds

So:

• распороть
• but, for comparison, разбудить

Not exactly.

Распороть is more specific than a general verb like разрезать (to cut). In sewing context, распороть means to open or undo something that has been sewn together, especially a seam.

So the idea is not just cutting, but unsewing / unpicking / ripping open along the stitching.

Here старый шов is in the accusative, because it is the direct object of the verb.

However, шов is a masculine inanimate noun. For masculine inanimate nouns in Russian, the accusative looks the same as the nominative.

So:

• nominative: старый шов
• accusative: старый шов

The adjective старый matches the noun, so it also stays the same here.

Compare that with an animate masculine noun:

• nominative: старый друг
• accusative: старого друга

You know mainly from meaning and normal word order.

The neutral order here is:

• subject + verb + object

So:

• Портной — the doer
• распорол — the action
• старый шов — the thing affected

It is true that шов could look nominative on the surface, because masculine inanimate accusative is identical to nominative. So in a sentence like this, learners rely on:

1. normal word order
2. common sense
3. the verb’s meaning

A tailor is much more likely to unpick a seam than the other way around.

The stresses are:

• портно́й
• распоро́л
• ста́рый
• шов

A rough pronunciation guide:

• портно́й — par-TNOY
• распоро́л — ras-pa-ROL
• ста́рый — STA-ryy
• шов — roughly shof

Two useful pronunciation notes:

• Unstressed о is reduced, so it often sounds closer to a or uh than a clear English o.
• Final в in шов is devoiced, so it sounds like f.

Yes. Russian word order is more flexible than English, because case endings do a lot of the grammatical work.

Your sentence:

• Портной распорол старый шов.

is the most neutral way to say it.

Other orders are possible, but they change the emphasis:

• Старый шов распорол портной. — emphasizes the tailor
• Старый шов портной распорол. — emphasizes the old seam

For beginners, the original subject-verb-object order is the easiest and most natural to use.

Russian has no articles.

So Russian does not directly mark the difference between:

• a tailor
• the tailor

or:

• an old seam
• the old seam

English chooses a/an/the from context, but Russian leaves that unstated unless something else makes it clear.

Шов is a masculine noun, and it has a stem change in most forms:

• nominative: шов
• genitive: шва
• dative: шву
• accusative: шов
• instrumental: швом
• prepositional: шве

Plural:

• nominative plural: швы

So yes, it is a normal masculine noun type overall, but you should notice the stem alternation:

• шов → шв-

Usually it literally means old: an older seam, probably one already there in the garment.

In sewing context, that makes good sense. A tailor may unpick an old seam before altering, repairing, or resewing something.

Depending on context, старый can sometimes feel close to previous or already existing, but the basic meaning is still old.