كان هذا المكتب كنيسة سابقا.

كان is the past-tense form of to be. In a sentence like this, it means was or sometimes used to be, depending on context.

So:

• هذا المكتب كنيسة = This office is a church
• كان هذا المكتب كنيسة = This office was a church / used to be a church

In Arabic, كان is one of the very common verbs that changes the grammar of the sentence that follows it.

Because the subject is المكتب (the office), and مكتب is a masculine noun.

Even though كنيسة (church) is feminine, the verb كان agrees with the subject, not with the predicate.

So:

• كان هذا المكتب... = correct, because المكتب is masculine
• كانت هذه الكنيسة... = would be correct if the subject were the church, which is feminine

هذا means this and is the masculine singular demonstrative.

It points to المكتب:

• هذا = this
• المكتب = the office

Together, هذا المكتب means this office.

In Arabic, the demonstrative usually comes before the noun, unlike English word order with some other structures.

Because هذا can stand alone as this, but if you want to say this office, you normally use both the demonstrative and the noun:

• هذا = this
• هذا المكتب = this office

The noun also usually keeps الـ when used after a demonstrative in this kind of phrase, so هذا المكتب is the normal pattern for this office.

المكتب is definite because it means the office, and it is also made specific by هذا (this).

كنيسة is indefinite because it is the predicate here: a church.

So the structure is:

• هذا المكتب = this office
• كنيسة = a church

That is why Arabic does not use الكنيسة here. If you said الكنيسة, it would mean the church, which changes the meaning.

Because كان changes the case pattern of the sentence.

With كان:

• the subject of كان stays nominative
• the predicate of كان becomes accusative

So in this sentence:

• هذا المكتب = the subject of كان
• كنيسةً = the predicate of كان, so it is accusative

In normal fully vowelled writing, you would see:

• كان هذا المكتبُ كنيسةً سابقًا

In everyday unvowelled writing, those endings are usually not written, but the grammar is still there.

سابقًا means previously, formerly, or before.

In this sentence, it tells you that the state of being a church belonged to the past.

So:

• كان هذا المكتب كنيسة سابقا = This office was previously a church / This office used to be a church

It functions like an adverb here.

That is a very natural place for it. Arabic often puts adverbial words like سابقًا after the main statement.

So the sentence first gives the main idea:

• كان هذا المكتب كنيسة = this office was a church

Then adds the time-related adverb:

• سابقًا = formerly / previously

You could also hear or see slight variations in word order for emphasis, but the final position is very common and natural.

مكتب can mean both:

• office
• desk

The correct meaning depends on context.

In this sentence, the intended meaning is clearly office, because This desk used to be a church would not make sense. So context tells you which meaning to choose.

Not a separate word, no. Arabic often expresses that idea through كان plus context.

So كان هذا المكتب كنيسة can mean:

• This office was a church
• This office used to be a church

Adding سابقًا makes the past/former meaning even clearer.

Without كان, you would get a nominal sentence in the present:

• هذا المكتب كنيسة = This office is a church

With كان, it shifts to the past:

• كان هذا المكتب كنيسة = This office was a church

So كان is what moves the sentence into the past.

Yes, to a limited extent. Arabic word order is somewhat flexible, especially with adverbs like سابقًا.

For example, these are possible:

• كان هذا المكتب كنيسةً سابقًا
• كان هذا المكتب سابقًا كنيسةً

Both are understandable. The first is probably the most straightforward for a learner.

But هذا المكتب should stay together as a unit meaning this office.

كنيسة is a feminine noun, and you can often tell because it ends in ـة (taa marbuuTa).

That matters for things like adjectives and pronouns that refer back to it, but in this sentence it does not control the verb. The verb agrees with المكتب, which is the subject.

So:

• المكتب = masculine subject
• كنيسة = feminine predicate

That is why the sentence still begins with كان, not كانت.