يوصي أمين المكتبة بموسوعة.

Because this is a normal verbal sentence in Modern Standard Arabic. Arabic often uses verb + subject + other elements.

So:

يوصي أمين المكتبة بموسوعة.
= The librarian recommends an encyclopedia.

This order is very natural in MSA.

You can also say:

أمين المكتبة يوصي بموسوعة.

That is also correct, but it starts with the subject instead, so it feels a bit more like The librarian is recommending an encyclopedia with slightly more focus on the librarian.

Here يوصي means recommends.

It is the imperfect/present form of أوصى.
This verb can have meanings like:

• to recommend
• to advise
• sometimes to instruct or to urge, depending on context

In this sentence, because of بـ before موسوعة, the meaning is to recommend.

يوصي is in the imperfect form. In Arabic, the imperfect can cover several English meanings depending on context, such as:

• recommends
• is recommending
• will recommend

Without extra context, the most natural translation here is recommends.

So the sentence can be understood as a general or present-time statement: The librarian recommends an encyclopedia.

Because this verb is commonly used with بـ when talking about the thing being recommended.

So:

أوصى / يوصي بـ...
= to recommend ...

That means موسوعة is not a direct object by itself here. It comes after the preposition بـ.

So literally, the structure is something like:

The librarian recommends with/regarding an encyclopedia

But in natural English, we simply say:

The librarian recommends an encyclopedia.

Because أمين المكتبة is an iḍāfa construction, often called a genitive construction or noun-noun construction.

Word by word:

• أمين = keeper, custodian, secretary, trustee
• المكتبة = the library

So literally it means:

the keeper/custodian of the library

In natural English, that is:

the librarian

So this is not just two separate nouns. They work together as one phrase.

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

So:

• أمين المكتبة = the librarian
• not الأمين المكتبة

The definiteness of the whole phrase comes from the second noun:

• المكتبة = the library
• therefore أمين المكتبة = the librarian

This is a very important pattern in Arabic.

Because it does not have الـ, so it means an encyclopedia, not the encyclopedia.

So:

• بموسوعة = with/an encyclopedia → natural English: an encyclopedia
• بالموسوعة = with the encyclopedia → natural English: the encyclopedia

So if you wanted The librarian recommends the encyclopedia, you would say:

يوصي أمين المكتبة بالموسوعة.

With full case endings, the sentence would be:

يُوصِي أَمِينُ الْمَكْتَبَةِ بِمَوْسُوعَةٍ

Here is the grammar:

• أمينُ → nominative, because it is the subject
• المكتبةِ → genitive, because it is the second part of an iḍāfa
• موسوعةٍ → genitive, because it comes after the preposition بـ

So even if short vowels are usually not written in normal Arabic texts, the underlying grammar is:

• subject = nominative
• after preposition = genitive
• second noun of iḍāfa = genitive

A careful pronunciation with case endings is:

yūṣī amīnu al-maktabati bi-mawsūʿatin

A more natural pause form, without fully pronouncing all final case endings, may sound closer to:

yūṣī amīn al-maktaba bi-mawsūʿa

A few helpful notes:

• يوصي = yūṣī
• موسوعة = mawsūʿa
• the letter ع in موسوعة is the consonant ʿayn, which has no exact English equivalent

Yes. You can also say:

أمين المكتبة يوصي بموسوعة.

This is also correct.

Both are grammatical, but the nuance is a little different:

• يوصي أمين المكتبة بموسوعة. → more classically verbal, very natural in MSA
• أمين المكتبة يوصي بموسوعة. → puts the subject first, so it gives a bit more prominence to the librarian

For a learner, it is useful to understand both patterns.