هذا أخي.

In Modern Standard Arabic, the verb to be is usually not expressed in the present tense in simple sentences like this.

So Arabic says:

هذا أخي
literally: this my-brother

but it means:

This is my brother.

If you wanted a past meaning, Arabic would use a verb, for example:

كان هذا أخي = This was my brother.

هذا means this.

It is the masculine singular demonstrative used for something or someone nearby. Since أخ means brother, which is masculine singular, هذا is the correct form.

Compare:

• هذا أخي = This is my brother
• هذه أختي = This is my sister

So the form of this changes depending on the gender of the noun.

أخي is made from:

• أخ = brother
• ـي = my

So:

أخ + ي = أخي
= my brother

The ـي ending is a attached pronoun meaning my.

This is very common in Arabic:

• كتابي = my book
• بيتي = my house
• اسمي = my name

Because أخي is already definite.

When a noun has a possessive ending like ـي (my), it is automatically definite. My brother already refers to a specific person, so Arabic does not add ال.

So:

• أخ = a brother / brother
• أخي = my brother

But not:

• الأخي ✘

There are two main reasons:

1. A noun with a possessive suffix like ـي does not take tanwīn.
2. With the my suffix, the ending you hear is -ī, so the case ending is not separately pronounced in the usual way.

So أخي stays أخي in normal use.

For a beginner, the important point is simply:

• أخي = my brother
• no ـٌ / ـٍ / ـً is added

It is pronounced approximately:

hādhā akhī

A few sounds to notice:

• ه = h
• ذ = like th in this
• خ = a rough sound, like the ch in German Bach or Scottish loch
• ي at the end of أخي gives a long ee sound

So:

هذا = hādhā
أخي = akhī

This is a nominal sentence in Arabic, meaning a sentence without a present-tense verb.

It has two parts:

• هذا = the subject/topic: this
• أخي = the predicate/information about it: my brother

So the structure is:

this + my brother
= This is my brother

This kind of sentence is extremely common in Arabic.

Yes, you can.

هذا هو أخي also means This is my brother.

The word هو here adds emphasis or clarity, somewhat like saying:

This is my brother indeed / This, he is my brother

But in simple everyday MSA, هذا أخي is completely correct and natural.

So:

• هذا أخي = basic, normal
• هذا هو أخي = also correct, slightly more emphatic

Not if you mean This is my brother.

أخي هذا is understood more like a noun phrase:

this brother of mine

That is different from the full sentence:

هذا أخي = This is my brother

So for the sentence you want, keep the order:

هذا أخي

Yes. In Arabic, demonstratives like هذا usually agree with the noun in gender and number.

Since أخي means my brother, and brother is masculine singular, the demonstrative is also masculine singular:

• هذا أخي = This is my brother

If the noun were feminine, you would change the demonstrative:

• هذه أختي = This is my sister

So agreement is an important part of Arabic grammar.