يقصر الخياط الكم.

A fully vocalized reading would most likely be:

يُقَصِّرُ الخَيَّاطُ الكُمَّ
yuqaṣṣiru al-khayyāṭu al-kumma

A few helpful notes:

• الخياط is pronounced al-khayyāṭ, with a doubled y sound.
• الكم is pronounced al-kumm, with a doubled m sound.
• In normal Arabic writing, short vowels and the shadda are often omitted, so learners have to know or infer the correct reading from context.

Because Arabic very often uses verb–subject–object order.

So this sentence is:

• يقصر = shortens
• الخياط = the tailor
• الكم = the sleeve

This gives Verb + Subject + Object.

Arabic can also use subject–verb–object:

الخياط يقصر الكم

That is also correct, but starting with the verb is very common in Modern Standard Arabic.

The subject is الخياط — the tailor.
The object is الكم — the sleeve.

So:

• يقصر = the action
• الخياط = the one doing the action
• الكم = the thing receiving the action

If the full case endings are shown, they help confirm this:

• الخياطُ = nominative, so it is the subject
• الكمَّ = accusative, so it is the direct object

The يـ marks the verb as third person masculine singular in the imperfect tense.

So يقصر means something like:

• he shortens
• he is shortening
• he shortens regularly

Arabic imperfect verbs can cover several present/future-style meanings depending on context.

In this sentence, the verb already contains the idea of he, so Arabic does not need a separate word for he.

This is a very good question, because the Arabic spelling without vowels can be ambiguous.

For the meaning shortens something, the expected reading is:

يُقَصِّرُ = he shortens

This is a Form II verb, with a doubled middle consonant.

But the same bare spelling يقصر could also be read as:

يَقْصُرُ = he is short / becomes short / falls short

So in an unvocalized sentence, the intended meaning comes from context. If the meaning shown to the learner is The tailor shortens the sleeve, then the intended reading is يُقَصِّرُ.

Because both nouns are definite:

• الخياط = the tailor
• الكم = the sleeve

The prefix الـ is the Arabic definite article, equivalent to the.

So the sentence is not saying just a tailor or a sleeve, but the tailor and the sleeve.

الخياط means the tailor, and it is a profession noun.

It comes from the root خ ي ط, which is connected with sewing and thread-related meanings.
A related word is:

• خيط = thread

So الخياط is literally the person associated with sewing.

الكُمّ means the sleeve.

Useful related information:

• singular: كُمّ
• plural: أكمام

So in this sentence, the tailor is altering the sleeve, specifically by shortening it.

In careful Modern Standard Arabic, especially in formal reading, the full endings would be:

• الخياطُ
• الكمَّ

However, in many real spoken contexts, speakers often pause or reduce case endings, especially outside very formal recitation or news-style speech.

So for learning purposes:

• it is good to know the endings
• but you may not always hear them clearly in actual usage

Yes, that is also grammatical.

Compare:

• يقصر الخياط الكم = verb-first, a very common Arabic structure
• الخياط يقصر الكم = subject-first, often used when the speaker wants to foreground the tailor

Both can mean the same thing, but the word order can slightly affect emphasis or style.