يقسم صديقي أن الكتاب هنا.

In Modern Standard Arabic, the present-tense is/are is usually not stated in simple nominal sentences.

So:

• الكتاب هنا literally looks like the book here
• but it means the book is here

This is completely normal Arabic.

If you wanted a past meaning, Arabic would usually use كان:

• كان الكتاب هنا = the book was here

Here it means that and introduces the clause the book is here.

So the structure is:

• يقسم صديقي = my friend swears
• أن الكتاب هنا = that the book is here

More precisely, when fully vocalized, this is أَنَّ rather than أَنْ. In ordinary Arabic writing, short vowels and the shadda are often omitted, so you just see أن.

Yes, in this sentence it is understood as أَنَّ.

You can tell from the grammar and meaning:

• أَنَّ introduces a clause meaning that...
• أَنْ usually goes before a verb and often means something like to or marks a subjunctive verb

Here, what follows is a nominal clause:

• الكتاب هنا = the book is here

So the intended reading is أَنَّ.

Yes. After أَنَّ, the noun immediately following it is grammatically accusative.

With full case endings, this would be:

• أَنَّ الْكِتَابَ هُنَا

But in normal unvocalized Arabic writing, both الكتابُ and الكتابَ are usually written simply as الكتاب.

So the case change is there grammatically, even though you do not see it in the spelling.

Because verb-first word order is very common in Arabic.

So:

• يقسم صديقي = my friend swears
• صديقي يقسم = my friend swears too

Both are possible, but they can feel slightly different in emphasis or style:

• يقسم صديقي... is a very normal verbal-sentence order
• صديقي يقسم... puts more attention on my friend

صديقي means my friend.

It is made of:

• صديق = friend
• -ي = my

So this is a noun plus a possessive suffix:

• صديقي = my friend

That final -ي is very common in Arabic:

• كتابي = my book
• بيتي = my house

It is in the imperfect form, which often covers meanings like:

• he swears
• he is swearing
• sometimes he does swear

The exact English translation depends on context. In a simple sentence like this, swears is the most natural choice.

That is a very good question, because the spelling يقسم is ambiguous without vowel marks.

It can be read in more than one way, including:

• يُقْسِمُ = he swears
• يَقْسِمُ = he divides / distributes

In this sentence, the meaning and structure show that it must be he swears.

So the intended pronunciation here is:

• يُقْسِمُ

هنا means here, and grammatically it is an adverb of place.

In the clause الكتاب هنا:

• الكتاب is the thing being talked about
• هنا tells the location

In traditional Arabic grammar, after أَنَّ, هنا functions as the predicate of the clause.

Also, هنا is an indeclinable word, so it does not take the usual case endings.

It would typically be written:

يُقْسِمُ صَدِيقِي أَنَّ الْكِتَابَ هُنَا

This shows a few useful things:

• يُقْسِمُ = he swears
• أَنَّ = that
• الْكِتَابَ is accusative after أَنَّ
• هُنَا = here

So the unvocalized sentence hides some grammar that becomes clear once the vowels are added.