هذه موسوعة في قسم المراجع.

هذه means this for a feminine singular noun. The word موسوعة (encyclopedia) is grammatically feminine, so the demonstrative has to match it.

• هذه موسوعة = This is an encyclopedia
• If the noun were masculine, you would normally use هذا

A very common clue for feminine nouns is the ending ـة (called tāʾ marbūṭa), which موسوعة has.

In Arabic, the verb to be in the present tense is usually not expressed. So a sentence like:

• هذه موسوعة

literally looks like this encyclopedia, but it means This is an encyclopedia.

This kind of sentence is called a nominal sentence. Arabic often leaves out is / am / are in the present.

موسوعة without ال means an encyclopedia, not the encyclopedia.

So:

• هذه موسوعة = This is an encyclopedia
• هذه الموسوعة = This is the encyclopedia

In your sentence, the speaker is identifying the object as one example of that category, not pointing it out as a specific already-known encyclopedia.

في means in.

So:

• في قسم المراجع = in the reference section

It is one of the most common Arabic prepositions.

Literally, قسم المراجع means the section of references.

This is a very common Arabic structure called an iḍāfa (possessive or construct phrase):

• قسم = section / department
• المراجع = the references

Together:

• قسم المراجع = reference section / references section

English often translates this more naturally rather than word-for-word.

This is because قسم المراجع is an iḍāfa construction.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun can be definite, and that definiteness makes the whole phrase definite

So:

• قسم المراجع = the reference section

Even though قسم itself has no ال, the whole phrase is definite because المراجع is definite.

It depends on context. قسم can mean section, department, or division.

In a library context, قسم المراجع is most naturally the reference section.

If this were talking about an institution or office, department might be a better translation.

In careful Modern Standard Arabic, هذه is pronounced roughly hādhihi.

A helpful breakdown is:

• ها = hā
• ذِ = dhi
• هِ = hi

In slower, careful speech you may hear the full hādhihi clearly. In many real-life pronunciations, especially outside very formal reading, speakers may reduce it somewhat, but for MSA learning, hādhihi is a good pronunciation target.

It is grammatically feminine because it ends in ـة (tāʾ marbūṭa), which is a very common marker of feminine nouns.

So Arabic treats it as feminine for agreement purposes. That is why you get:

• هذه موسوعة

and not:

• هذا موسوعة

Even when a noun refers to an object and not a female person, it can still be grammatically feminine.

Yes, but the meaning or emphasis may shift.

The basic sentence:

• هذه موسوعة في قسم المراجع

means This is an encyclopedia in the reference section.

You might also hear or see variations depending on what the speaker wants to emphasize, but for learners, the given order is a very natural and straightforward one:

• هذه = this
• موسوعة = an encyclopedia
• في قسم المراجع = in the reference section

So it is best to learn this as a standard nominal sentence with a prepositional phrase at the end.

It breaks down like this:

• هذه = demonstrative pronoun (this)
• موسوعة = predicate noun (an encyclopedia)
• في قسم المراجع = prepositional phrase (in the reference section)

So the sentence pattern is:

• this + noun + prepositional phrase

More grammatically, it is a nominal sentence:

• هذه is the topic/start of the sentence
• موسوعة gives the identification
• في قسم المراجع adds location

Not exactly in its current form. هذه موسوعة most naturally means This is an encyclopedia.

If you want This encyclopedia is in the reference section, Arabic would more naturally be:

• هذه الموسوعة في قسم المراجع

because الموسوعة means the encyclopedia.

So the presence or absence of ال makes an important difference:

• هذه موسوعة = This is an encyclopedia
• هذه الموسوعة = This encyclopedia / This is the encyclopedia

No, not fully. In normal Arabic writing, short vowel endings are usually omitted.

A fully vowelled formal version could show case endings, but in ordinary text you usually just see:

• هذه موسوعة في قسم المراجع

without the final grammatical vowels written out.

For most learners at an early stage, it is enough to understand the sentence without worrying too much about the hidden case endings.