هو يرخي البرغي بمفك براغي.

Because هو is optional here.

In MSA, يرخي by itself already means he loosens / he is loosening, so Arabic does not need a separate subject pronoun the way English usually does. The sentence could simply be:

يرخي البرغي بمفك براغي.

Adding هو can:

• make the subject extra clear,
• add emphasis,
• sound natural if the speaker is contrasting him with someone else.

So هو is not wrong; it is just not strictly necessary.

يرخي is in the imperfect tense/form in Arabic.

In MSA, the imperfect can express:

• a present action: he is loosening
• a habitual action: he loosens
• sometimes a near future meaning, depending on context

So without extra context, يرخي can mean either he loosens or he is loosening.

The dictionary form is أرخى.

So the pair is:

• أرخى = the past/basic dictionary form
• يرخي = the imperfect form

In this sentence, يرخي means to loosen. With something mechanical like البرغي, the idea is to loosen a screw.

Because it has الـ, the definite article, so it means the screw rather than a screw.

• البرغي = the screw
• برغي or برغيًا = a screw (depending on case and whether endings are written)

Arabic often uses definiteness very explicitly, so if the speaker has a particular screw in mind, البرغي is the normal choice.

Here بـ means with or by means of.

So:

• بمفك براغي = with a screwdriver

This is a very common use of بـ in Arabic: it can mark the instrument used to do something.

Literally, the structure is:

• بِ
  • مفك = with a screwdriver

Because Arabic often expresses this kind of idea as an iḍāfa construction, sometimes called a construct phrase.

Here:

• مفك = a turning/undoing tool
• براغي = screws

So مفك براغي literally means something like a tool for screws, which is the Arabic way of saying screwdriver.

English often packs this into one word, but Arabic commonly uses two nouns together instead.

Because in an iḍāfa construction, the first noun normally does not take الـ.

So:

• مفك براغي = a screwdriver
• مفك البراغي = the screwdriver

The definiteness of the whole phrase is usually determined by the second noun.

That is why مفك appears without الـ.

That is just the idiomatic Arabic expression.

Arabic often names tools as a tool for X, and the second noun may naturally appear in the plural. So مفك براغي is the standard kind of expression for screwdriver, even though English uses singular screw in the compound.

So do not translate it too literally word by word. Treat مفك براغي as a set expression meaning screwdriver.

In full formal grammar:

• البرغي is the direct object, so it is accusative
• مفك comes after بـ, so it is genitive
• براغي is the second part of an iḍāfa, so it is also genitive

However, in normal Arabic writing, these short case endings are usually not written, and in everyday pronunciation they are often not fully pronounced either.

So a learner will usually see the sentence simply written as:

هو يرخي البرغي بمفك براغي

without visible case vowels.

Yes. Arabic allows more than one normal word order.

For example:

• هو يرخي البرغي بمفك براغي.
• يرخي البرغي بمفك براغي.

Both are possible.

Very generally:

• verb-first is very common in Arabic
• subject-first can give a slightly more explicit or emphatic feel

So the sentence with هو is fine, but Arabic often omits the pronoun and starts directly with the verb.

Often, yes, depending on the exact nuance.

• يرخي focuses on loosening
• يفك often gives the sense of undoing / unscrewing / taking apart

With a screw, both can make sense in context, but they are not always identical in nuance. If the idea is specifically making the screw less tight, يرخي fits well. If the idea is unscrewing or undoing it, يفك may be more natural in many contexts.

A useful approximate pronunciation is:

huwa yurkhī al-burghī bi-mifakk barāghī

A few pronunciation notes:

• kh is the throaty sound heard in Scottish loch or German Bach
• مفك has a doubled k sound because of the shadda: mifakk
• يرخي ends with a long -ī sound

So you can say it roughly as:

huwa yur-khee al-bur-ghee bi-mi-fakk ba-raa-ghee